Handling private communication of subcommittees

[jakirkham]

Currently we state in the governance document that we would like subcommittees to discuss items that must be discussed in private using the steering council mailing list. However this raises a concern as subcommittees can have non-steering council members, who would now have access to steering council member conversation. This would be good to address, but is a bit tricky as it could have a larger maintenance cost associated. Raising here to break this out of the governance discussion.

cc @mmccarty @mrocklin (who were engaged in this thread)