misc: Add a code of conduct

misc: Add a code of conduct

This file codifies our current unofficial policies. This doesn't change
the community in any way except for clarifying our policies.

This was modeled off of https://www.contributor-covenant.org/.

Change-Id: Ib976636b490bbe4d46bf79260e6a345b46c02e2c
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30954
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>