Document how changes flow between maintained branches

[desrosj]

💡 Suggestion

The Contributing guide makes note of the three major versions of Canvas Kit that are supported at a given point and which branches those live in (though 4 are listed below the sentence preceding the list). However, it's not clear how changes flow between these branches after being merged.

It seems like .github/workflows/forwarrd-merge.yml handles this automatically. But this is not obvious unless you understand the YAML that GitHub Actions workflow files use.

It would be great to have this documented within the Contributing guide so it's easier to understand.

Motivation

To make it easier to contribute to Canvas Kit.

Example

I had something like this in mind:

• Changes merged to x-branch will be merged to y-branch by the forward-merge workflow provided the PR has the Z label/automatically after being merged/is fixes: commit (whatever the conditions are)
• Changes merged to y-branch are never merged into x-branch.
• etc.