Document running the background worker.

[jsha]

This is necessary for uploaded crates to properly display their READMEs.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @carols10cents (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[JohnTitor]

r? @JohnTitor @bors r+

[bors]

📌 Commit 3261a83 has been approved by JohnTitor

[bors]

⌛ Testing commit 3261a83 with merge 36a469a...

[bors]

☀️ Test successful - checks-travis
Approved by: JohnTitor
Pushing 36a469a to master...