Update CONTRIBUTING.md
Update CONTRIBUTING.md

Add link to "How to Contribute to Open Source" guide.
Unidata/MetPy,ahaberlie/MetPy,ShawnMurd/MetPy,jrleeman/MetPy,Unidata/MetPy,dopplershift/MetPy,dopplershift/MetPy,jrleeman/MetPy,ahaberlie/MetPy