DOC: Add a call for contribution paragraph to the readme
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.

madphysicist/numpy,endolith/numpy,WarrenWeckesser/numpy,jorisvandenbossche/numpy,pizzathief/numpy,grlee77/numpy,rgommers/numpy,charris/numpy,seberg/numpy,numpy/numpy,mattip/numpy,abalkin/numpy,pbrod/numpy,jorisvandenbossche/numpy,charris/numpy,mhvk/numpy,mattip/numpy,madphysicist/numpy,rgommers/numpy,simongibbons/numpy,madphysicist/numpy,numpy/numpy,jorisvandenbossche/numpy,simongibbons/numpy,endolith/numpy,MSeifert04/numpy,anntzer/numpy,pbrod/numpy,anntzer/numpy,anntzer/numpy,simongibbons/numpy,jakirkham/numpy,grlee77/numpy,simongibbons/numpy,endolith/numpy,simongibbons/numpy,pizzathief/numpy,MSeifert04/numpy,madphysicist/numpy,grlee77/numpy,pdebuyl/numpy,jakirkham/numpy,jorisvandenbossche/numpy,mhvk/numpy,seberg/numpy,mhvk/numpy,anntzer/numpy,MSeifert04/numpy,grlee77/numpy,charris/numpy,pizzathief/numpy,mattip/numpy,rgommers/numpy,pdebuyl/numpy,jorisvandenbossche/numpy,pdebuyl/numpy,WarrenWeckesser/numpy,endolith/numpy,WarrenWeckesser/numpy,pbrod/numpy,MSeifert04/numpy,numpy/numpy,pizzathief/numpy,mhvk/numpy,pbrod/numpy,rgommers/numpy,abalkin/numpy,mhvk/numpy,numpy/numpy,pizzathief/numpy,charris/numpy,pdebuyl/numpy,jakirkham/numpy,ahaldane/numpy,abalkin/numpy,WarrenWeckesser/numpy,jakirkham/numpy,WarrenWeckesser/numpy,jakirkham/numpy,madphysicist/numpy,grlee77/numpy,MSeifert04/numpy,ahaldane/numpy,ahaldane/numpy,seberg/numpy,ahaldane/numpy,mattip/numpy,pbrod/numpy,seberg/numpy,ahaldane/numpy