Add instructions for serving web site locally and contributing

[kbsezginel]

These instructions should include how to install ruby, jekyll, serve the website, make changes and open a pull request.

[tahirozdemir34]

We can update CONTRIBUTING.md like this: https://gist.github.com/tahirozdemir34/dc46424c377db266adaaa09862b573b4 (installation instructions tested on all 3 systems)

or maye we can create wiki pages for the project for instructions.

[kbsezginel]

I think editing CONTRIBUTING.md sounds good, would be great if you could open a pull request :) I just opened a new develop branch you can directly commit to that branch and open the pull request from there.