Document CNCF Slack and responsible disclosure

[jayofdoom]

Adds a couple of new, useful sections to contributing document:

• Adds information about the CNCF Slack server, where #armada exists now.
• Adds a section about responsible security disclosure.

[jayofdoom]

My testing of [email protected] earlier today showed it was not provisioned yet, so we should not merge this until we have confirmed that is working as expected.