Add guide on how to run a different CI job for a try build

[Kobzol]

Here is a diff for example: https://gist.github.com/Kobzol/66bcd8f9c4153803c25c46966d306c13

[rustbot]

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