Add bors and rust-highfive support to this repo.

[michaelwoerister]

Instructions by @Mark-Simulacrum and @pietroalbini from discord:

• bors will be via https://github.com/rust-lang/rust-central-station/blob/master/homu.toml.template; highfive you need to add a file to here https://github.com/rust-lang/highfive/tree/master/highfive/configs/rust-lang and then ping @pietroalbini or me, we can finish setup

• for bors you also need to add the configuration in the team repo for the perms

• likely adding another bors.foo.review = true to https://github.com/rust-lang/team/blob/master/teams/compiler.toml#L22; note that the foo there is the bors name that you use in the homu toml file
  (which can be the same as repo name, but doesn't have to be)

• and adding foo to the src/permissions.rs
  I should really spend some time to document stuff