[Docs] Add ISSUE_TEMPLATE

Add ISSUE TEMPLATE to nntrainer
- github.sec.samsung.net automatically copies your commit message for your convenience.
- Please write your own PR description with this template and remove unused part of the template.

**Self evaluation:**
1. Build test:	 [X]Passed [ ]Failed [ ]Skipped
2. Run test:	 [X]Passed [ ]Failed [ ]Skipped

Signed-off-by: Donghak PARK <[email protected]>

.github/ISSUE_TEMPLATE.md

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+---
+name: Issue report
+about: Create a report to help us improve
+
+---
+
+Having problems with a source code of a github repository?
+
+Having problems with the CI bot that controls the build process?
+
+Good to go? Then please remove these lines above, including this one, and help us understand your issue by answering the following:
+
+# Issue Description
+A clear and concise description of what the bug is.
+
+Expected Result
+============
+A clear and concise description of what you expected to happen.
+
+How to Reproduce
+===============
+1. Go to '...'
+2. Click on '....'
+3. Scroll down to '....'
+4. See error
+
+
+Further Information
+===============
+* A link to an output result showing the issue
+* Exact OS version

.github/ISSUE_TEMPLATE/Bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+
+---
+
+Having problems with a source code of a github repository?
+
+Having problems with the CI bot that controls the build process?
+
+Good to go? Then please remove these lines above, including this one, and help us understand your issue by answering the following:
+
+# Issue Description
+A clear and concise description of what the bug is.
+
+Expected Result
+============
+A clear and concise description of what you expected to happen.
+
+How to Reproduce
+===============
+1. Go to '...'
+2. Click on '....'
+3. Scroll down to '....'
+4. See error
+
+
+Further Information
+===============
+* A link to an output result showing the issue
+* Exact OS version

.github/ISSUE_TEMPLATE/Feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+
+---
+
+**Is your feature request related to a problem? Please describe.**
+A clear and concise description of what the problem is. For example, I'm always frustrated when [...]
+
+**Describe the solution you'd like**
+A clear and concise description of what you want to happen.
+
+**Describe alternatives you've considered**
+A clear and concise description of any alternative solutions or features you've considered.
+
+**Additional context**
+Add any other context or screenshots about the feature request here.

.github/ISSUE_TEMPLATE/Support_request.md

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+---
+name: Support Request
+about: Report a problem with our project source code
+
+---
+
+![WARNING](https://media.giphy.com/media/Zsx8ZwmX3ajny/giphy.gif)
+
+Please only create issues/feature requests for the project here.
+
+For support contact our project maintainer(s), they meet online in a 'Issues' list.
+There you can ask questions if you have trouble understanding something, seek advice and mingle with other project members.
+For further information see 'Wiki' page.

.github/PULL_REQUEST_TEMPLATE.md

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+
+---
+# [Template] PR Description
+
+In general, github system automatically copies your commit message for your convenience.
+Please remove unused part of the template after writing your own PR description with this template.
+```bash
+$ git commit -s filename1 filename2 ... [enter]
+
+Summarize changes in around 50 characters or less
+
+More detailed explanatory text, if necessary. Wrap it to about 72
+characters or so. In some contexts, the first line is treated as the
+subject of the commit and the rest of the text as the body. The
+blank line separating the summary from the body is critical;
+various tools like `log`, `shortlog` and `rebase` can get confused 
+if you run the two together.
+
+Further paragraphs come after blank lines.
+
+**Changes proposed in this PR:**
+- Bullet points are okay, too
+- Typically a hyphen or asterisk is used for the bullet, preceded
+  by a single space, with blank lines in between, but conventions vary here.
+
+Resolves: #123
+See also: #456, #789
+
+**Self evaluation:**
+1. Build test: [ ]Passed [ ]Failed [*]Skipped
+2. Run test: [ ]Passed [ ]Failed [*]Skipped
+
+**How to evaluate:**
+1. Describe how to evaluate in order to be reproduced by reviewer(s).
+
+Add signed-off message automatically by running **$git commit -s ...** command.
+
+$ git push origin <your_branch_name>
+```
+

.github/SECURITY.md

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+# Security policy
+
+1. [Vulnerability Reports Table](#1-vulnerability-reports-table)  
+2. [Vulnerability Report](#2-vulnerability-report)  
+3. [Security Disclosure](#3-security-disclosure)  
+
+---
+
+## 1. Vulnerability Reports Table
+
+Table reports on vulnerabilities found and patches/descriptions to mitigating them.
+| Version     | Vulnerability | Patch/description |
+| ----------- | --------------|-------------------|
+| 1.x.x       | N/A           |                   |
+| 2.x.x       | N/A           |                   |
+
+---
+
+## 2. Vulnerability Report
+
+Search and fix of vulnerability issue is the highest priority for the NNStreamer project team.
+
+Please report security bugs by contact to [jaeyun-jung](https://github.com/jaeyun-jung) marked "SECURITY".
+NNstreamer team will confirm your request and within 2 week will try to prepare recommendations for elimination. Our team will keep you updated on the progress towards the fix until the full announcement of the patch release. During this process, the NNStreamer team may request additional information or guidance.
+
+---
+
+## 3. Security Disclosure
+
+When a person responsible for security receives a vulnerability report as previously mentioned, it is assigned the highest priority and the person in charge. This person will coordinate the patch and release process.
+
+Actions that must be made by the NNStreamer team.
+  * Confirm the problem and identify the affected versions.
+  * Check the code to find any similar problems.
+  * Prepare fixes for all releases still in maintenance. These fixes will
+    released as quickly as possible.
+
+We suggest the following format when disclosing vulnerabilities:
+
+  * Your name and email.
+  * Include scope of vulnerability. Let us know who could use this exploit.
+  * Document steps to identify the vulnerability. It is important that we can reproduce your findings. 
+  * How to exploit vulnerability, give us an attack scenario.