Add more documentation to readme
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 <d95b56ce41a2e1ac4cecdd398defd7414407cc08@causes.com>
Reviewed-by: Aiden Scandella <1faf1e8390cdac9204f160c35828cc423b7acd7b@causes.com>

gnagel/overcommit,TheSooth/overcommit,oxocode/overcommit,TheSooth/overcommit,blech75/overcommit,gnagel/overcommit,oxocode/overcommit,blech75/overcommit