RFR: Code of conduction added

Mon Jul 1 11:32:53 UTC 2019

I added a code of conduction that is based on http://contributor-covenant.org
I think it makes always sense to have such file and with OpenJDK moving to GitHub it could easily be added to each repo. 

Only open point is the team mail address that should be added to the document (see TODO_MAIL_ADRESS). If you add the mail address as a comment in this PR I can add it.

----------------

Commits:
 - 67b603b3:	Code of conduction added

Pull request:
http://git.openjdk.java.net/skara/pull/17

Webrev:
https://openjdk.github.io/cr/skara/17/webrev.00

Patch:
http://git.openjdk.java.net/skara/pull/17.diff

Fetch command:
git fetch https://github.com/openjdk/skara.git 67b603b3:pr/17