add a contributing.md

Author: timfel

CONTRIBUTING.md

@@ -0,0 +1,44 @@
+**Thank you for thinking about contributing something to this Python
+implementation on Graal!**
+
+## First things first
+
+You will need to sign the [Oracle Contributor
+Agreement](http://www.graalvm.org/community/contributors/) for us to be able to
+merge your work.
+
+Please also take some time to review our [code of
+conduct](http://www.graalvm.org/community/conduct/) for contributors.
+
+## How to contribute?
+
+If you want to make a contribution, it's best to open an issue first about the
+area you would like to offer your help with. This implementation is in an early
+state and it will make more sense to work on some areas than others, so before
+you sink work into a pull request that we won't be able to accept, please talk
+to us :)
+
+To get started with development on Python, first make sure you can build at
+least the _truffle_ project in the [Graal](https://github.com/oracle/graal)
+repository with our build tool [mx](https://github.com/graalvm/mx), as well as
+[Sulong](https://github.com/graalvm/sulong).
+
+Once you can build those projects, clone this repository and run
+
+    $ mx build
+    $ mx python -c "print(42)"
+
+If this prints "42", then everything went fine and you just built and ran
+Python. Note that you don't need GraalVM for this, but then you'll only run
+interpreted, which won't give very good performance (but that might be fine for
+development).
+
+When you make your changes, you can test them with `mx python`. Additionally,
+there are various "gates" that we use on our CI system to check any code that
+goes in. You can run all of these gates using `mx python-gate` or just some by
+using `mx python-gate --tags [TAG]`. Two interesting tags to run that cover most
+things are:
+
+- python-unittest
+- python-graalvm
+- python-license