Add Cargo backport instructions

Author: alexcrichton

beta-backporting.md

@@ -1,6 +1,15 @@
+---
+layout: default
+title: Beta Backporting · The Rust Programming Language
+---
+
+# Beta Backporting
+
 There's a steady trickle of patches that need to be ported to the beta
 branch. Only a few people are even aware of the process, but this is
-actually something anybody can do. Here's how it works:
+actually something anybody can do.
+
+## Backporting in `rust-lang/rust`
 
 When somebody identifies a PR that should be backported to beta they
 tag it
@@ -19,15 +28,40 @@ ready for a backport is those tagged [both `beta-nominated` and
 
 So now somebody needs to go through those PR's and cherry-pick their
 commits to the beta branch. Those cherry-picks are then submitted as a
-PR _against the beta branch_, with a title started with `beta` (so
+PR _against the beta branch_, with a title started with `[beta]` (so
 reviewers can see its specialness). The OP of that PR should contain
 links to all the PRs being backported. [Here's an
 example](https://github.com/rust-lang/rust/pull/36634). Anybody can
 make these PRs!
 
 After that a reviewer needs to verify that the backport looks correct,
-that it's submitted to the beta branch, and then hit the merge
-button. Finally, they need to follow the links to the original PRs and
-_remove the `beta-nominated` tag_ (people forget to do this a
-lot). This last step indicates that the backport has been completed,
-so the `beta-nominated` and `beta-accepted` tags have three states.
+that it's submitted to the beta branch, and then approve via `@bors: r+`.
+Finally, they need to follow the links to the original PRs and _remove the
+`beta-nominated` tag_ (people forget to do this a lot). This last step indicates
+that the backport has been completed, so the `beta-nominated` and
+`beta-accepted` tags have three states.
+
+## Backporting in `rust-lang/cargo`
+
+The procedure for backporting fixes to Cargo is similar bug slightly more
+extended than the `rust-lang/rust` repo's procedure. Currently there aren't
+backport tags in the Cargo repository, but you'll initiate the backport process
+by commenting on an associated PR, requesting a backport. Once a Cargo team
+member has approved the backport to happen you're good to start sending PRs!
+
+* First you'll send a PR to the `rust-1.21.0` branch of Cargo (replace 1.21 with
+  the current rustc beta version number). Like with `rust-lang/rust` you'll
+  prefix the title of your PR with `[beta]` and ensure it's flagged as going to
+  beta.
+
+* Next a Cargo reviewer will `@bors: r+` the PR and put it into the queue.
+  Eventually bors will automatically merge the PR (when tests are passing) to
+  the appropriate Cargo branch.
+
+* Finally you'll send a PR to the `rust-lang/rust` repository's `beta` branch,
+  updating the Cargo submodule. The Cargo submodule should be updated to the tip
+  of the `rust-1.21.0` branch (the branch your Cargo PR was merged to). As like
+  before, ensure you've got `[beta]` in the PR title.
+
+After that's all said and done the Cargo change is ready to get scheduled onto
+the beta release!