Add mergify rule to auto merge backported PR

Signed-off-by: Madhu Rajanna <madhupr007@gmail.com>
(cherry picked from commit 136d81b736)

.mergify.yml

@@ -19,3 +19,27 @@ pull_request_rules:
         strict: smart
       dismiss_reviews: {}
       delete_head_branch: {}
+  - name: backport patches to release branch
+    conditions:
+      - base=master
+      - label=backport-to-release-v1.2.0
+    actions:
+      backport:
+        branches:
+          - release-v1.2.0
+  # automerge backports if CI successfully ran
+  - name: automerge backport release-v1.2.0
+    conditions:
+      - author=mergify[bot]
+      - base=release-v1.2.0
+      - label!=DNM
+      - "#changes-requested-reviews-by=0"
+      - "#approved-reviews-by>=1"
+      - "status-success=continuous-integration/travis-ci/pr"
+    actions:
+      merge:
+        method: rebase
+        rebase_fallback: merge
+        strict: smart
+      dismiss_reviews: {}
+      delete_head_branch: {}

docs/development-guide.md

@@ -89,3 +89,13 @@ need to be met before it will be merged:
 
 When the criteria are met, a project maintainer can merge your changes into
 the project's master branch.
+
+The flow for getting a fix into a release branch is:
+
+1. Open a PR to merge the changes to master following the process outlined above.
+1. Add the backport label to that PR such as `backport-to-release-vX.Y.Z`
+1. After your PR is merged to master, the mergify bot will automatically open a
+   PR with your commits backported to the release branch
+1. If there are any conflicts you will need to resolve them by pulling the
+   branch, resolving the conflicts and force push back the branch
+1. After the CI is green, the bot will automatically merge the backport PR.