contrib guide: add PR, code review sections and cleanup
contrib guide: add PR, code review sections and cleanup

This patch hits a couple of improvement needed areas:
* add a pull requests section
* add a code review section
* misc smaller cleanups noticed while doing the prior two

Signed-off-by: Tim Pepper <4807564e5a18784afafcf20dbdb5591e844b9075@vmware.com>

bgrant0607/community,kubernetes/community,bgrant0607/community,pwittrock/community,kubernetes/community,kubernetes/community,pwittrock/community