Add mergify configuration

Author: okurz

.mergify.yml

@@ -0,0 +1,33 @@
+pull_request_rules:
+  - name: automatic merge
+    conditions:
+      - and: &base_checks
+          - base=master
+          - -label~=^acceptance-tests-needed|not-ready
+          - status-success=integration
+          - status-success=unit
+      - and:
+          - "#approved-reviews-by>=1"
+          - "#changes-requested-reviews-by=0"
+          # https://doc.mergify.io/examples.html#require-all-requested-reviews-to-be-approved
+          - "#review-requested=0"
+    actions:
+      merge:
+        method: merge
+  - name: automatic merge on special label
+    conditions:
+      - and: *base_checks
+      - and:
+          # mergify config checks needs at least two rules in "and" so we repeat
+          # one from the base checks
+          - base=master
+          - label=merge-fast
+    actions:
+      merge:
+        method: merge
+  - name: ask to resolve conflict
+    conditions:
+      - conflict
+    actions:
+      comment:
+        message: This pull request is now in conflicts. Could you fix it? 🙏