DOC: Add a call for contribution paragraph to the readme

We would like to attract contributions from people that are currently
underrepresented in the developer team. In particularly with regards
to skills in high level documentation or website.
As an attempt to reach more users, add this to the very visible
README.

Briefly wrote this paragraph with Ralf, we can iterate on it.

README.md

@@ -35,4 +35,17 @@ Tests can then be run after installation with:
 
     python -c 'import numpy; numpy.test()'
 
+
+Call for Contributions
+----------------------
+
+NumPy appreciates help from a wide range of different backgrounds. In particular
+work such as high level documentation or website improvements are incredibly
+valuable and we would really like to grow our team with people filling these roles.
+Small improvements or fixes are always appreciated and issues labeled as easy
+may be a good starting point.
+If you are considering larger contributions outside the traditional coding work,
+please do get in contact with us for example through the mailing list.
+
+
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