Better /contributing page on cosr-about

[sylvinus]

The current one is a first draft, very incomplete.

A few inspirations:

• https://www.mozilla.org/en-US/contribute/
• https://wiki.mozilla.org/Mobile/Get_Involved
• https://swift.org/contributing/
• https://www.drupal.org/getting-involved
• https://wiki.ubuntu.com/ContributeToUbuntu

[sylvinus]

First round of improvements: https://about.commonsearch.org/contributing