Update contributing.md document to reflect our current process (#5719)
Update contributing.md document to reflect our current process (#5719)

* Update contributing.md document to reflect our current process

- Get rid of references to GitHub issues
- Document our backporting policy
- Small addition to test policy
- How to install mesos; soften test requirement
- Update running integration test instructions

JIRA Issues: MARATHON-7262



janisz/marathon,gsantovena/marathon,meln1k/marathon,janisz/marathon,mesosphere/marathon,guenter/marathon,meln1k/marathon,mesosphere/marathon,guenter/marathon,janisz/marathon,mesosphere/marathon,gsantovena/marathon,gsantovena/marathon,guenter/marathon,mesosphere/marathon,meln1k/marathon,gsantovena/marathon,janisz/marathon,janisz/marathon,meln1k/marathon,guenter/marathon,mesosphere/marathon,gsantovena/marathon,guenter/marathon,meln1k/marathon