README.md says:

Save the Web, Write Some Tests!

Absolutely everyone is welcome to contribute to test development. No
test is too small or too simple, especially if it corresponds to
something for which you've noted an interoperability bug in a browser.

The way to contribute is just as usual:

• Fork this repository (and make sure you're still relatively in sync
  with it if you forked a while ago).
• Create a branch for your changes:
  git checkout -b topic.
• Make your changes.
• Run ./wpt lint as described above.
• Commit locally and push that to your repo.
• Create a pull request based on the above.

CONTRIBUTING.md says:

All contributions are licensed under the terms of the 3-Clause BSD License.

Documentation

See web-platform-tests.org.

Because CONTRIBUTING.md is linked from some GitHub UI, all the information should probably go there and be referenced from README.md.