Contribution policy addition request

[rvagg]

To those here that are obsessive about avoiding merge commits please expand the contribution policy with technical details about how to land patches to (1) avoid merge commits and (2) keep a healthy git log in the process. I find the process kind of infuriating and have messed up the commit log slightly while landing the last few PRs.

The contribution policy is also lacking technical details about how committers should be keeping contributions to a single commit, ammending to insert the requested tags, and other minor details; it's a bit too hand wavy for a project that has built up an rigid set of conventions about how to git.

[ralphtheninja]

👍

[indutny]

@rvagg should be fixed by #68, please follow the steps when landing it ;)