This does not need to be a full-fledged document. Describe the dimensions of tests that this feature is expected to pass when it is done. If it needs to be tested with different hardware or software configurations, indicate them. The more specific you can be, the better the community testing can be.
Remember that you are writing this how to for interested testers to use to check out your feature - documenting what you do for testing is OK, but it's much better to document what *I* can do to test your feature.
A good "how to test" should answer these four questions:
- What special hardware / data / etc. is needed (if any)?
- How do I prepare my system to test this feature? What packages need to be installed, config files edited, etc.?
- What specific actions do I perform to check that the feature is working like it's supposed to?
- What are the expected results of those actions?