Backport PR when label is set

backport PR to release-v1.2.0 branch
from master branch  when label is set
on PR

Signed-off-by: Madhu Rajanna <madhupr007@gmail.com>

.mergify.yml

@@ -19,3 +19,11 @@ 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

docs/development-guide.md

@@ -89,3 +89,10 @@ 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.
+
+### Backporting a PR to release branch
+
+If the PR needs to be backported to a release branch, a project maintainer adds
+`backport-to-release-vX.Y.Z` label to the PR. Mergify bot will take care of
+sending the backport PR to release branch once the PR is merged in the master
+branch.