Add mergify configuration (#465)

Author: La0

.mergify.yml

@@ -0,0 +1,23 @@
+---
+pull_request_rules:
+  - name: automatic merge for Dependabot pull requests
+    conditions:
+      - author~=^dependabot(|-preview)\[bot\]$
+      - status-success=Community-TC (pull_request)
+      - status-success=FirefoxCI (pull_request)
+      - base=master
+      - "#approved-reviews-by>=1"
+    actions:
+      merge:
+        method: squash
+
+  - name: Automatic merge on approval for contributors
+    conditions:
+      - author!=@mozilla/sallt
+      - status-success=Community-TC (pull_request)
+      - status-success=FirefoxCI (pull_request)
+      - base=master
+      - "#approved-reviews-by>=1"
+    actions:
+      merge:
+        method: squash