Document how to add a new content provider

[choldgraf]

Proposed change

We should document how to add a new content provider to repo2docker (similar to jupyterhub/binderhub#1017). I can't currently find this information in the documentation, and I think it's important that it is accessible given that this is one of the primary ways that repo2docker can be extended.

Who can do this work?

Someone familiar with the process to add a new content provider. If anybody who has gone through the process wants to give a short bulleted list I can flesh it out into a docs PR.