Clarify contribution expectations

Makes a checklist of contribution expectations with links to the original documents. The docs are currently pretty spread out and it is difficult for a new person to be aware of all of the things they are supposed to consider. This clarifies the expectations without re-architecting the docs.
Should we mention `git commit --amend`? My usual process is:

 1. submit PR
 1. get comments
 1. update pr, do `got commit --amend`
 1. force push. Github does the right thing to the PR in the web ui