Add contribution guidelines

Signed-off-by: Denis Varlakov <[email protected]>

CONTRIBUTING.md

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+# Contributing Guide
+
+Thanks for taking interest to contributing to our project!
+
+## Join us in Discord!
+We welcome all contributors to communicate with us [in Discord]! Please, reach out to us
+in `#lockness-contribute` room.
+
+## Pull Requests
+Prior to making a PR, we ask you to communicate it with us, either [in Discord] or, if you
+prefer, by opening an issue in the repo. This would help to keep your work aligned with the
+maintainers view and avoid situations in which we can't accept your contribution.
+
+All commits are required to be signed via verified GPG key. You can read about commit signing
+in [this series of articles](https://docs.github.com/en/authentication/managing-commit-signature-verification)
+(we recommend using a hardware GPG token).
+
+All commits are required to be signed off by including `Signed-off-by: YOUR NAME <[email protected]>` line.
+By doing this, you certify that the commit is compliant with [Developer Certificate of Origin (DCO)](https://developercertificate.org/),
+meaning that you wrote the code or otherwise have the right to submit the code you are
+contributing to the project.
+
+```text
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.
+```
+
+Commits can be automatically signed off automatically by using `-s` flag (i.e. `git commit -s`).
+
+## Issues
+Feel free to open an issue if you found a bug, have a suggestion, or wish to
+communicate with us for other reasons.
+
+However, if you want to report something that you believe might be a security
+vulnerability or a security flaw in this or any upstream project, please report
+it following the procedure described in [SECURITY.md](./SECURITY.md).
+
+Feel free to reach out to us [in Discord] as well.
+
+[in Discord]: https://discordapp.com/channels/905194001349627914/1285268686147424388