add mergable changelog test

.github/mergeable.yml

@@ -0,0 +1,21 @@
+version: 2
+mergeable:
+  - when: pull_request.*
+    name: "Changelog check"
+    validate:
+        - do: or
+          validate:
+            - do: dependent
+              changed:
+                file: 'src/**' 
+                required: ['CHANGELOG.md']
+            - do: dependent
+              changed:
+                file: 'deltachat-ffi/**'
+                required: ['CHANGELOG.md']
+    fail:
+      - do: checks
+        status: 'action_required' 
+        payload:
+          title: Changelog needs to be updated
+          summary: "Please update the changelog"