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 
I think this is actually not whate recommend normally. I think somewhere else we already say that we prefer a github install with symlinks