add auto backport workflow (opensearch-project#172)

Signed-off-by: Amit Galitzky <amgalitz@amazon.com>

Author: amitgalitz

.github/workflows/backport.yml

@@ -0,0 +1,16 @@
+name: Backport
+on:
+  pull_request:
+    types:
+      - closed
+      - labeled
+
+jobs:
+  backport:
+    runs-on: ubuntu-latest
+    name: Backport
+    steps:
+      - name: Backport
+        uses: tibdex/backport@v1
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}

DEVELOPER_GUIDE.md

@@ -79,3 +79,11 @@ yarn prettier --write <relative file path>
 ```
 
 > NOTE: There also exists prettier plugins on several editors that allow for automatic reformatting on saving the file. However using this is discouraged as you must ensure that the plugin uses the correct version of prettier (listed in `package.json`) before using such a plugin.
+
+### Backports
+
+The Github workflow in [`backport.yml`](.github/workflows/backport.yml) creates backport PRs automatically when the
+original PR with an appropriate label `backport <backport-branch-name>` is merged to main with the backport workflow
+run successfully on the PR. For example, if a PR on main needs to be backported to `1.x` branch, add a label
+`backport 1.x` to the PR and make sure the backport workflow runs on the PR along with other checks. Once this PR is
+merged to main, the workflow will create a backport PR to the `1.x` branch.