Adopt a platform today!

[David Chisnall]

Some time around the last release, there was an idea that we should document 
the platforms that we actually care about in the next release.  If you care 
about a platform, then you should:

- Run the test suite on that platform
- Report (or, ideally) fix failures
- Be the contact point for bug reports that are specific to that platform

It would be nice if we could actually do this and advertise in the next release:

Tier 1 platforms, where someone is running the tests and is willing to fix bugs.
Tier 2 platforms, where someone has run the test in the past and we think stuff 
still works, but no one is actively maintaining, and
Tier 3 platforms (everything else - may work, but you're on your own).

David