Add link to Contributing.md
Add link to Contributing.md

This links to a good video series on making GitHub contributions.
ahill818/MetPy,Unidata/MetPy,ahaberlie/MetPy,Unidata/MetPy,jrleeman/MetPy,jrleeman/MetPy,dopplershift/MetPy,dopplershift/MetPy,ahaberlie/MetPy,ShawnMurd/MetPy,deeplycloudy/MetPy