Create GOVERNANCE.md

[Bisaloo]

This version focuses on documenting existing structure and processes, and making them publicly explicit.

It has been reviewed by the leadership of the different organizations. Review at this time is mostly focused on checking the Google doc -> GitHub-flavoured markdown conversion.

[chartgerink]

This port over looks in good shape 😊

I left a few suggestions around sorting names alphabetically (easier to find folk) and around the figure. Consider those at your own convenience, and not required for merging (I did not raise these before either).