Update the instructions on setting up bors

Author: rylev

src/infra/docs/bors.md

@@ -21,10 +21,7 @@ failed are re-approved again on their own.
 
 There are multiple steps needed to add a repository to our Bors instance:
 
-1. The [@bors] GitHub account needs to be granted write access to the
-   repository.
-
-2. Each CI provider needs to have a single GitHub Check Run to gate on. This is
+1. Each CI provider needs to have a single GitHub Check Run to gate on. This is
    not provided by default on GitHub Actions, but it can be simulated with
    these two jobs, which will generate a `bors build finished` check:
 
@@ -51,23 +48,43 @@ There are multiple steps needed to add a repository to our Bors instance:
    Make sure to replace `[ALL, OTHER, JOBS]` with a list of all the jobs you
    want to gate on.
 
-3. Add the repository name to the `permissions!` macro in the [team
+   These jobs need to run on specific branches (`auto` and `try`) so it's necessary
+   to add those branches to the list of branches tested by the CI provider. For GitHub
+   Actions that looks like this:
+
+   ```yaml
+   on:
+      push:
+          branches: [ 
+            master, # This is any branch you normally run CI on
+            auto,   # Added for bors
+            try     # Added for bors
+         ]
+   ```
+
+2. Add the repository name to the bors permissions array in the [team
    repository][team-permissions.rs], and grant the `bors.REPOSITORY.review`
-   permission to the right people.
+   permission to the right teams or people. You can see an example of adding
+   bors permissions to a team [here][bors-permission].
 
 3. Add the repository to the `repositories` map in [the Terraform configuration
    file][tf-repos]. This will create a webhook and inject its secret key in the
    bors execution environment.
 
 4. Add the repository to the [Bors configuration][bors-config], taking
-   inspiration from other repositories.
+   inspiration from other repositories. Note that the environment variables used
+   in that config will be set automatically as long as you completed step 3 above.
+
+5. Give it a test by commenting `@bors ping` in any PR. If you get a response back,
+   you can then try to approve the PR with `@bors r+`.
 
 [@bors]: https://github.com/bors
 [Homu]: https://github.com/rust-lang/homu
 [bors-config]: https://github.com/rust-lang/homu/blob/master/cfg.production.toml
 [bors.rust-lang.org]: https://bors.rust-lang.org
 [ecs]: ./ecs-service.md
 [rust-lang/homu]: https://github.com/rust-lang/homu
-[team-permissions.rs]: https://github.com/rust-lang/team/blob/master/src/permissions.rs
+[team-permissions.rs]: https://github.com/rust-lang/team/blob/52b4370214e1c8eabe483f3a26f22733d94b326f/config.toml#L18-L37
+[bors-permission]: https://github.com/rust-lang/team/blob/a1532ec2b08c9d40c0a2c7643ffe72de9671e265/teams/wg-compiler-performance.toml#L25-L26
 [tf-repos]: https://github.com/rust-lang/simpleinfra/blob/master/terraform/bors/_config.auto.tfvars
 [tf]: https://github.com/rust-lang/simpleinfra/tree/master/terraform/bors/