ENH: Add section on contributing to README
ENH: Add section on contributing to README

Inspired by Adrienne Lowe's PyCon 2016 talk, add some language encouraging people not to be afraid to contribute.
ahill818/MetPy,dopplershift/MetPy,ahaberlie/MetPy,Unidata/MetPy,ShawnMurd/MetPy,jrleeman/MetPy,jrleeman/MetPy,dopplershift/MetPy,Unidata/MetPy,ahaberlie/MetPy