CONTRIBUTING.md: add guide for first-time contributors

Getting started contributing to Git can be difficult on a Windows
machine. CONTRIBUTING.md contains a guide to getting started, including
detailed steps for setting up build tools, running tests, and
submitting patches to upstream.

[includes an example by Pratik Karki how to submit v2, v3, v4, etc.]

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>