Add contributing file

Author: damacus

CONTRIBUTING.md

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+# Contributing
+
+## Branches
+
+### `master` branch
+
+The master branch is the current committed changes. These changes may not yet be released although we try to release often.
+
+## Tags
+
+All releases are tagged in git. To see the releases available to you see the changelog or the tags directly.
+
+## Pull requests
+
+- <https://github.com/sous-chefs/nginx/pulls>
+
+## Issues
+
+Need to report an issue? Use the github issues:
+
+- <https://github.com/sous-chefs/nginx/issues>