new section 'call to action'

proposal:

1. add new section called 'Call to action'
2. Pick one item from https://github.com/search?q=type:issue+org:racket+state:open+label:%22good+first+issue%22
3. call on new contributors to check it out and close the issue with a PR.
4. honour new contributors in racket news! (you already do that!)

Maybe https://github.com/racket/math/issues/9 for the first one?

What do you think?

maybe the first 'call to action' should be calling on contributors to identify existing issues labeled 'good first issue' and submitting them here?