add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+Contributions are **welcome**. Please report any issue you find in the [issues page](https://github.com/AndikanGabriel/coindesk/issues). [Pull requests](https://github.com/AndikanGabriel/coindesk/pulls) are more than welcome. Please review these guidelines before submitting any pull requests.
+
+## Guidelines
+
+* Please follow the **[PSR-12 Coding Standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-12-extended-coding-style-guide.md)**.
+* Ensure that the current **tests** pass, and if you've added something new, add the tests where relevant.
+* Remember that we follow **[SemVer](http://semver.org)**. If you are changing the behaviour, or the public API, you may need to update the `README.md` and other relevant documentation.
+* Send a coherent commit history, making sure each individual commit in your pull request is meaningful. If you had to make multiple intermediate commits while developing, please [squash](http://git-scm.com/book/en/Git-Tools-Rewriting-History) them before submitting.
+* Please **create feature branches** for your pull requests.
+* **One pull request per feature** - If you want to do more than one thing, send multiple pull requests.
+* You may also need to [rebase](http://git-scm.com/book/en/Git-Branching-Rebasing) to avoid merge conflicts.