DOC: add doc/source/contributing.rst as the CONTRIBUTING.md
xref #9754 

I just did

`pandoc doc/source/contributing.rst -t markdown > CONTRIBUTING.md`

to generate the new CONTRIBUTING file.

cc @rockg
@jorisvandenbossche 
 ok, so this is the question. I can edit this file, but then these are disconnected.

anyone used pandoc for this before?
