DOC Add CONTRIBUTING.md file

CONTRIBUTING.md

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+# Contributing
+
+Pull requests are welcome, but often it will be fruitful to first discuss it on [the
+mailing-list](https://ngless.embl.de/contacts.html).
+
+Generally, making all the tests pass is needed for a pull request to be accepted, although
+sometimes a pull request can be in the form of a failing test (as a bug report). Note that
+NGLess has a strong culture of strict backwards compatibility, so changing behaviour
+requires (at the very least) discussion.
+
+For all but the smallest changes, please list them on the `ChangeLog` and the
+`docs/source/whatsnew.rst` files.