Add information about image tests to dev docs.
Add information about image tests to dev docs.

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