Plugin documentation added

[rg663]

Resolves issue #472 — Documentation to link a Django app, or plugin, to ColdFront is added to the ColdFront documentation at https://coldfront.readthedocs.io/en/latest/.

[rg663]

@dsajdak I edited the documentation to illustrate adding the plugin to the plugins directory, which is definitely much more organized than adding it to the core directory. I actually was testing it in the core directory on my end for simplicity in the beginning, so I forgot to change it to the plugins directory when writing the docs!