Add the Backport GitHub Action workflow, which automatically backports PRs when you add the backport label (#62)

* Add the `Backport` GitHub Action workflow, which automatically backports PRs when you add the backport label

* Enable the `Backport` workflow for pull requests that were made from forks

See tibdex/backport#77 for details

Author: DilumAluthge

.github/workflows/backport.yml

@@ -0,0 +1,16 @@
+name: Backport
+on:
+  pull_request_target:
+    types:
+      - closed
+      - labeled
+jobs:
+  backport:
+    runs-on: ubuntu-18.04
+    name: Backport
+    if: github.event.pull_request.merged
+    steps:
+      - name: Backport
+        uses: tibdex/backport@v1
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}