New boilerplate for crater results

[aidanhs]

Initial minimal tweak:

Hi X (crater requester), Y (reviewer)! Crater results are at: <url>. 'Blacklisted' crates (spurious failures etc) can be found here. If you see any spurious failures not on the list, please make a PR against that file.

[aidanhs]

Minor alteration after consulting infra team.

Hi X (crater requester), Y (reviewer)! Crater results are at: <url>. 'Blacklisted' crates (spurious failures etc) can be found [here](https://github.com/rust-lang-nursery/crater/blob/master/blacklist.md). If you see any spurious failures not on the list, please make a PR against that file.

(interested observers: Crater is a tool for testing the impact of changes on the crates.io ecosystem. You can find out more at the [repo](https://github.com/rust-lang-nursery/crater/) if you're curious)

[pietroalbini]

This needs to be updated with the new blacklisting?