[TB] Create a SIG for Theme Builder

[aaronreed708]

Problem/Concern

Suggested to @lwnoble by attendees at All Things Open: we should have a SIG (special interest group) for Theme Builder where people could engage with each other and ask questions and share ideas.

Proposed Solution

Apparently this is something through GitHub. Lise and Aaron to research and bring back proposal to Theme Builder community.

[aaronreed708]

Something like this, maybe? https://github.com/finos/open-regtech-sig. Seems to just be a repo and a mailing list.

Or this? https://github.com/cdfoundation/sig-security Seems to be a repo, mailing list and Slack.