Add contributor guide in README

[YJDave]

Add guidelines for contributors to contribute to loklak.
Some suggestions are:

• Add reference to good articles on contribution to FOSS projects
• Add guide on how to start server and how to run tests
• Add guide on what is good PR? how to create issues? How to write commit message?

[YJDave]

@daminisatya FYI, if you can add more points here.

[daminisatya]

@YJDave Can you go through susi_server and loklak repos to check how the guidelines are posted?

[YJDave]

Okay, @daminisatya Thanks for suggestion! I will check that out.