Add Mergify rule to delete merged branch and bors+

.mergify.yml

@@ -0,0 +1,23 @@
+pull_request_rules:
+  - name: instruct bors to merge PRs with passing tests and 2 approvals
+    conditions:
+      - "#approved-reviews-by>=2"
+      - "#changes-requested-reviews-by=0"
+      - "status-success=static_analysis"
+      - "-status-failure~=^build"
+      - -conflict
+      - label!=work-in-progress
+      - label!=blocked
+      - label!=no-mergify
+      - head~=^(?!release.*).*$
+      - base=dev
+    actions:
+      comment:
+        message: "bors r+"
+  - name: Delete branch if the pull request is merged
+    conditions:
+      - merged
+      - head~=^(?!release.*).*$
+    actions:
+      delete_head_branch:
+        force: false