DOC: improve contributing.html documentation

Tell new contributors about:

* whats_new.rst
* python_reference.rst
* make html-noplot
* python setup.py develop --user

my 2c towards https://github.com/mne-tools/mne-python/issues/1495 
@dengemann you don't need with develop to rerun setup.py. It's really symlinks. We just discovered this after writing this first doc.