README: Link to CONTRIBUTING.md

Make that information more easily discoverable.  And since
CONTRIBUTING.md already mentions IRC, we can drop the IRC reference
from the README to DRY things up.

Also update CONTRIBUTING.md to replace the stale #cri-o reference left
over from the initial libpod/podman fork.  While I was touching this
line, I also shuffled some of the wording around to tighten that
sentence up.

Signed-off-by: W. Trevor King <wking@tremily.us>

Closes: #687
Approved by: rhatdan
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