Also add CoC paragraph (copied from CONTRIBUTING) to the README.

Signed-off-by: Paul Jolly <paul@myitcv.io>
Change-Id: Ic22fc44ba8433a81ec2e3466f51eb6a0139b5f31
Reviewed-on: https://review.gerrithub.io/c/cue-labs/oci/+/1176981


TryBot-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>