Update readme for contributing.
Update readme for contributing.

ahaberlie/MetPy,dopplershift/MetPy,jrleeman/MetPy,ahill818/MetPy,Unidata/MetPy,Unidata/MetPy,dopplershift/MetPy,jrleeman/MetPy,deeplycloudy/MetPy,ShawnMurd/MetPy,ahaberlie/MetPy