More substantive readme

Now that we're answering some of the same questions about how to use this repo, it's probably a good time to put some of this in the README. :bulb: 

There's enough rearranging the deck chairs here that it's probably easiest to review [rendered version](https://github.com/railsbridge/organizing/blob/rachelmyers-patch-1/README.md). 

Let me know if you have feedback! :smile_cat: 

:ocean: