Review "Using Apache Airflow Plugins" + Add Airflow Plugins Repo Link

[jwitz]

@paolaperaza commented on Thu Apr 11 2019

We got some feedback from a customer on our "Using Apache Airflow Plugins" Guide that he'd like to see more context on there.

It would be helpful l to have end to end context on how to build and use custom plugins. I want able to find external examples as well because we can just add external link to the end of this doc and that should suffice.

We should look over this doc + reformat some of it + beef it up if we can, and at the very least link out to our Airflow Plugins repo.