Branch policies and strategy

[yizha1]

We have adopted branch policies recently for sub projects, see one of the issues notaryproject/notation#519.

This issue is for formally document the branch policies and strategy, like:

Manage main branch

• Keep a high quality, up-to-date main branch.

Manage features

• Use feature branches to develop new features, and naming convention
• How to merge feature branches into the main branch using pull requests.

Manage releases

• Use release branches to coordinate and stabilize changes in a release
• Lifecycle management of release branch, for example, lock release branches when we are ready to stop supporting a particular release.

Manage bugs fixes

• Use branches for bug fixes if need?

Define the rules for each branch. Rules could be different in some cases for different branches. Here are the rules we already applied for some repos.

• Require PR before merging
• Require number of approvals before merging, the current number for most sub projects is 3 approvals.
• Dismiss stale PR approvals when new commits are pushed
• Require review from Code Owners
• Require status checks to pass before merging
• Require conversation resolution before merging
• Require signed commits
• Do not allow bypass the above settings