Add matomo update docs

docs/upgrade-matomo.md

@@ -0,0 +1,13 @@
+# Upgrading Matomo 
+
+In order to upgrade Matomo, follow these steps:
+
+- Change the version in the Ansible config like this: https://github.com/stadtnavi/digitransit-ansible/commit/f5d3b6cdc6d9cbab9025758994b7f050ef3cd3a2
+- Provision the server with `make dev` or `make prod`
+- Then log into Matomo and migrate in the UI
+    - This will attempt a database migration but this may time out. That's not particularly bad. Just follow the next step.
+- Make sure that the database is migrated with the following commands:
+  ```
+  docker exec -it matomo_app_1 /bin/sh
+  php /var/www/html/console core:update
+  ```