doc: add guide for backporting prs

This guide should help answer questions for contributors
that are not familiar with the backport process.

COLLABORATOR_GUIDE.md

@@ -337,7 +337,8 @@ LTS release.
 
 When you send your pull request, consider including information about
 whether your change is breaking. If you think your patch can be backported,
-please feel free to include that information in the PR thread.
+please feel free to include that information in the PR thread. For more
+information on backporting, please see the [backporting guide][].
 
 Several LTS related issue and PR labels have been provided:
 
@@ -364,3 +365,5 @@ When the LTS working group determines that a new LTS release is required,
 selected commits will be picked from the staging branch to be included in the
 release. This process of making a release will be a collaboration between the
 LTS working group and the Release team.
+
+[backporting guide]: doc/guides/backporting-to-release-lines.md

doc/guides/backporting-to-release-lines.md

@@ -0,0 +1,84 @@
+# How to backport a Pull Request to a Release Line
+
+## Staging branches
+
+Each release line has a staging branch that the releaser will use as a scratch
+pad while preparing a release. The branch name is formatted as follows:
+`vN.x-staging` where `N` is the major release number.
+
+### Active staging branches
+
+| Release Line | Staging Branch |
+| ------------ | -------------- |
+| `v7.x`       | `v7.x-staging` |
+| `v6.x`       | `v6.x-staging` |
+| `v4.x`       | `v4.x-staging` |
+
+## What needs to be backported?
+
+When a release is being prepared, the releaser attempts to cherry-pick a
+certain set of commits from the master branch to the release staging branch.
+The criteria for consideration depends on the target version (Current vs. LTS).
+If a cherry-pick does not land cleanly on the staging branch, the releaser
+will mark the pull request with a particular label for that release line,
+specifying to our tooling that this pull request should not be included. The
+releaser will then add a comment that a backport is needed.
+
+## What can be backported?
+
+The "Current" release line (currently v7.x) is much more lenient than the LTS
+release lines in what can be landed. Our LTS release lines
+(currently v4.x and v6.x) require that commits live in a Current release for at
+least 2 weeks before backporting. Please see the [`LTS Plan`][] for more
+information. After that time, if the commit can be cherry-picked cleanly from
+master, then nothing needs to be done. If not, a backport pull request will
+need to be submitted.
+
+## How to submit a backport pull request
+
+For these steps, let's assume that a backport is needed for the v7.x release
+line. All commands will use the v7.x-staging branch as the target branch.
+In order to submit a backport pull request to another branch, simply replace
+that with the staging branch for the targeted release line.
+
+* Checkout the staging branch for the targeted release line
+* Make sure that the local staging branch is up to date with the remote
+* Create a new branch off of the staging branch
+
+```shell
+# Assuming your fork of Node.js is checked out in $NODE_DIR,
+# the origin remote points to your fork, and the upstream remote points
+# to git://github.com/nodejs/node
+cd $NODE_DIR
+git checkout v7.x-staging
+git pull -r upstream v7.x-staging
+# We want to backport pr #10157
+git checkout -b backport-10157-to-v7.x
+```
+
+* After creating the branch, apply the changes to the branch.
+
+```shell
+git cherry-pick $SHA # Use your commit hash
+```
+
+* The commit message should be as close as possible to the commit message on the
+  master branch, unless the commit has to be different due to dependencies that
+  are not present in the targeted release line.
+* Push the changes to your fork and open a pull request.
+* Be sure to target the `v7.x-staging` branch in the pull request.
+
+### Helpful Hints
+
+* Please include the backport target in the pull request title in the following
+  format: `(v7.x backport) <commit title>`
+  * Ex. `(v4.x backport) process: improve performance of nextTick`
+* Please include the text `Backport of #<pull request number>` in the
+  pull request description. This will link to the original pull request.
+
+In the event the backport pull request is different than the original,
+the backport pull request should be reviewed the same way a new pull request
+is reviewed. When each commit is landed, the new reviewers and the new PR-URL
+should be used.
+
+[`LTS Plan`]: https://github.com/nodejs/LTS#lts-plan