Merge branch 'ReadabilityOfContributingDoc' into 'master'

Extension for markdown should improve readability

It was hard to read this online.  This should enable links where
appropriate.

Unfortunately, I am not positive that this won't break some tool relying on the filename not having an extension.

See merge request !771

CONTRIBUTING → CONTRIBUTING.md

-You can help to improve the www.gitlab.com website by sending merge requests to this repository. Thank you for your help!
+You can help to improve the [www.gitlab.com](www.gitlab.com) website by sending merge requests to this repository. Thank you for your help!
 By submitting code as an individual you agree to the [individual contributor license agreement](doc/legal/individual_contributor_license_agreement.md).
 By submitting code as an entity you agree to the [corporate contributor license agreement](doc/legal/corporate_contributor_license_agreement.md).