enable mergify (#553)

.mergify.yml

@@ -0,0 +1,20 @@
+pull_request_rules:
+  - name: automatic merge marked PRs on CI success and review
+    conditions:
+      # All Azure builds should be green:
+      - "status-success=digital-asset.daml"
+        # CLA check must pass:
+      - "status-success=license/cla"
+      # Special label to request automerge:
+      - "label=automerge"
+      # Note that this only matches people with write / admin access to the repo,
+      # see <https://doc.mergify.io/conditions.html#attribute-list>
+      - "#approved-reviews-by>=1"
+      # Note: mergify cannot break branch protection rules
+    actions:
+      merge:
+        method: squash
+        strict: smart
+        strict_method: rebase
+      # Takes no options, see <https://doc.mergify.io/examples.html#deleting-merged-branch>
+      delete_head_branch: {}