Add help wanted badge to readme

[carolynvs]

What does this change

The badge counts the number of help wanted and good first issues that are open and unassigned.

What issue does it fix

Closes #1635

Notes for the reviewer

Checklist

• Did you write tests?
• Did you write documentation?
• Did you change porter.yaml or a storage document record? Update the corresponding schema file.
• If this is your first pull request, please add your name to the bottom of our Contributors list. Thank you for making Porter better! 🙇‍♀️

Reviewer Checklist

• Comment with /azp run test-porter-release if a magefile or build script was modified
• Comment with /azp run porter-integration if it's a non-trivial PR