backports: mergify (#6107)

Co-authored-by: Tess Rinearson <tess.rinearson@gmail.com>

.github/mergify.yml

@@ -8,3 +8,11 @@ pull_request_rules:
         method: squash
         strict: true
         commit_message: title+body
+  - name: backport patches to v0.34.x branch
+    conditions:
+      - base=master
+      - label=S:backport-to-v0.34.x
+    actions:
+      backport:
+        branches:
+          - v0.34.x

CONTRIBUTING.md

@@ -289,6 +289,8 @@ the backport branches have names like `v0.34.x` or `v0.33.x` (literally, `x`; it
 
 As non-breaking changes land on `master`, they should also be backported (cherry-picked) to these backport branches.
 
+We use Mergify's [backport feature](https://mergify.io/features/backports) to automatically backport to the needed branch. Depending on which backport branch you need to backport to there will be labels for them. To notify the bot to backport a pull request, mark the pull request with the label `backport-to-<backport_branch>`. Once the original pull request is merged, the bot will try to cherry-pick the pull request to the backport branch. If the bot fails to backport, it will open a pull request. The author of the original pull request is responsible for solving the conflicts and merging the pull request.
+
 Minor releases don't have release candidates by default, although any tricky changes may merit a release candidate.
 
 To create a minor release: