Add more documentation to readme

- Start diving into plugin architecture
- Mention non-Ruby hooks
- Fix github URL (no www. prefix)

Change-Id: I0128e67ae94e2b736a11ef215bd4eb01155e0323
Reviewed-on: https://gerrit.causes.com/22794
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Aiden Scandella <aiden@causes.com>