Auto merge of #10916 - ehuss:contrib-shortcuts, r=weihanglo
Auto merge of #10916 - ehuss:contrib-shortcuts, r=weihanglo

Contrib: Add docs on the rustbot ready command

This adds some brief docs on the use of ``@rustbot`` [shortcuts](https://github.com/rust-lang/triagebot/wiki/Shortcuts).

eminence/cargo,rust-lang/cargo,rust-lang/cargo,nipunn1313/cargo,eminence/cargo,rustyhorde/cargo,rustyhorde/cargo,rustyhorde/cargo,nipunn1313/cargo,rust-lang/cargo,nipunn1313/cargo,rust-lang/cargo,eminence/cargo,eminence/cargo,nipunn1313/cargo,rustyhorde/cargo