Add triagebot configuration

[Mark-Simulacrum]

This enables assignment through triagebot on this repository, in preparation for the migration from highfive to triagebot for PR assignment.

cc rust-lang/highfive#258 rust-lang/triagebot#433

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @mookid (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 Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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.

[mookid]

@Mark-Simulacrum thanks!