Be very clear about Redmine's existence.

People sometimes get irritated when they open an issue in GitHub issues only to have it closed out.

This change hopes to point people at Redmine early on in the CONTRIBUTING.md, which pops up when people visit GitHub Issues.

I wish I could use blink and marquee tags on that too-subtle pointer of, "Please review the guidelines for contributing to this repository," but that won't happen any time soon.

## Verification

 - [x] Read the first three paragraphs of CONTRIBUTING.md
 - [x] Become convinced that Redmine is a thing
 - [x] Click on the link and see that you're on the Framework project.