Write M2 contribution guidelines

[SubPointSupport]

As M2 sometimes developed by other people, a contribution guide is a must.
The following areas are to be covered:

• VS setup, version, tooling
• SharePoint setup
• Solution structure, codebase structure
• What can be done, how to close a ticket
• Running integration tests
• Pushing changes

Octokit's project guide is a nice one, can be used as a started to get M2 covered:
https://github.com/octokit/octokit.net/blob/master/CONTRIBUTING.md

Moved from SubPointSolutions/spmeta2#785