Create CONTRIBUTING.md (fenom-template#2)

.github/CONTRIBUTING.md

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+Contributing
+============
+
+Issue tracker
+-------------
+
+The Issue tracker serves mainly as a place to report bugs and request new features.
+Please do not abuse it as a general questions or troubleshooting location.
+
+For these questions you can always use the
+[fenom tag](https://stackoverflow.com/questions/tagged/fenom) at [Stack Overflow](https://stackoverflow.com/).
+
+* Please provide a small example in php/html that reproduces your situation
+* Please report one feature or one bug per issue
+* Failing to provide necessary information or not using the issue template may cause the issue to be closed without consideration.
+
+Pull requests
+-------------
+
+Pull requests should be always based on the default [development](https://github.com/fenom-template/fenom/tree/master)
+branch except for backports to older versions.
+
+Some guidelines:
+
+* Use an aptly named feature branch for the Pull request.
+
+* Only files and lines affecting the scope of the Pull request must be affected.
+
+* Make small, *atomic* commits that keep the smallest possible related code changes together.
+
+* Code should be accompanied by a unit test testing expected behaviour.
+
+When updating a PR, do not create a new one, just `git push --force` to your former feature branch, the PR will
+update itself.