docs: add contributing guidelines

For some time now we the maintainers have been requesting things like
50/72 commit messages, unit tests, Tested: footers, etc. But these
requirements haven't been written down. This document is an attempt to
codify some of the unwritten rules of contribution in the OpenBMC IPMI
stack.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Change-Id: Ieb7a72f89f44374efbbe7265c7341338f6850158
1 file changed