feat: add branch-merge action

.github/workflows/branch-merge.yml

+name: Merge Branch
+
+on:
+  push:
+    branches: [ imgbot ]
+
+jobs:
+  merge-branch:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: everlytic/branch-merge@1.1.0
+        with:
+          github_token: ${{ github.token }}
+          source_ref: ${{ github.ref }}
+          target_branch: 'master'
+          commit_message_template: '[Automated] Merged {source_ref} into {target_branch}'