Update readme with frontend/yaml instructions

[castrojo]

We'll use the README in this repo to iterate over instructions for submission, etc. and that will act as a prototype for the "about" page for the front end.

Additionally the readme should include reviewer instructions so that people understand how we accept contributions, guidelines, etc.

[cjyabraham]

We should make it clear that presentations from the Community site will automatically be added and should not be added manually.