doc: describe the project's decision-making process

doc: describe the project's decision-making process

The Git project currently operates according to an informal
consensus-building process, which is currently described in the
SubmittingPatches document. However, that focuses on small/medium-scale
patch series. For larger-scale decisions, the process is not as well
described. Document what to expect so that we have something concrete to
help inform newcomers to the project.

This document explicitly does not aim to impose a formal process to
decision-making, nor to change pre-existing norms. Its only aim is to
describe how the project currently operates today.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>