Document regarding the rules for managing pull requests

[David Quintana (gigaherz)]

Team members and other interested parties,

At this month's meeting, we decided we had to have a talk about the growing
number of open pull requests. From such a discussion, we game up with a set
of rules that the team members should follow, when reviewing or managing
PRs.

If you are such a person, please take a moment to make sure you can agree
with those rules. There's a PR waiting for your input:

https://github.com/reactos/reactos/pull/585