Add doc for displaying branch-specific badge

[tribbloid]

I wonder if this is a feature that you have implemented, but didn't announce in the readme file?

If I use the branch argument in the URL, this plugin can generate the badge for that specific branch. E.g.

https://github-actions.40ants.com/40ants/ci/matrix.svg?branch=support-asdf-version

Generates the badge:

Should it be added? PR can come immediately

[svetlyak40wt]

Yeah, I forgot to mention this in the docs :(

[svetlyak40wt]

Feel free to send a pull-request.