Document SourceCred's plugin system and how to add new plugins

Going through SourceCred’s codebase to try to better understand how things work. This is difficult as most stuff isn’t documented. What is documented, however, are the 2019 OKRs. One of the items on the list was to document the plugin system and how people can add new ones. Seems pretty important, esp as we start to engage in outreach efforts.

Starting this thread to discuss how we can start to document the plugin system and what might be involved in that effort :slight_smile: