Update README.md

Added sequence diagram concerning joining projects.

I really like your project and would like to help.  While reading through the description, I built a sequence diagram to help with clarifying the details.  This was done with [websequencediagrams.com](websequencediagrams.com), but it could be done with more open technology using [js-sequence-diagrams](https://github.com/bramp/js-sequence-diagrams).  I chose the hosted, proprietary format to avoid committing binary image files to the repository.

Please let me know if you would like me to proceed with this.