Add governance document based on CNCF subproject governance template.
The first commit is an unchanged copy of the original CNCF template from 3 years ago (more recent updates of the template itself are not covered in this PR and can be seen this diff)
The second commit updates the template to fit the TUF org and was contributed by @joshuagl also 3 years ago (see commit message for details)
The third commit was contributed by @JustinCappos based on recent community discussions. Most notably, it adds two new TUF project types and describes a process where the steering committee members are elected by TUF contributors (previously the steering committee was constituted from subproject representatives, TAP editors and the consensus builder without an election)
Add governance document based on CNCF subproject governance template.
This PR is meant as discussion starter.