code-of-conduct.rst: Add Stefano Stabellini as a Conduct Team member

code-of-conduct.rst: Add Stefano Stabellini as a Conduct Team member

With my upcoming leave, Ian will be the only person actively on the
Conduct Team.  Stefano has volunteered to join the team, so that there
are at least two members.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
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Voting results:

* Hypervisor Team
- 7 members, quorum 3
- Votes: +1 x6 (of which 1 was +2, counted as +1 for voting purposes)
- 6 non-zero votes; quorum achieved
- Total approval: 100%

* XAPI team
- 5 members, quorum 2
- Votes: +1 x 3 (of which 1 was +2, counted as +1 for voting purposes)
- 3 non-xero votes; quorum achieved
- Total approval: 100%

* Windows PV driver team
- 2 members, quorum 1
- No votes; quorum not achieved

Vote average: (100 + 100) / 2 == 100% approval, the proposal passes.