chore(matrix_alertmanager): add role metadata

roles/matrix_alertmanager/meta/main.yml

@@ -0,0 +1,12 @@
+---
+allow_duplicates: true
+dependencies: []
+galaxy_info:
+  role_name: matrix_alertmanager
+  description: Deploy matrix_alertmanager, an alertmanager receiver for matrix rooms
+  galaxy_tags:
+    - matrix_alertmanager
+    - alertmanager_receiver
+    - alertmanager
+    - matrix
+    - docker