Add IRC channel to README.rst
Add IRC channel to README.rst

A IRC channel has been made where discussion can be performed
about networkx, using the popular opensource freenode servers
so it might as well be mentioned in the README.rst so people
can find it as needed.

andnovar/networkx,SanketDG/networkx,goulu/networkx,NvanAdrichem/networkx,Sixshaman/networkx,JamesClough/networkx,cmtm/networkx,jfinkels/networkx