docs(conf): adds docs about supporting database_url

closes #1878

docs/sources/installation/configuration.md

@@ -144,6 +144,10 @@ Grafana needs a database to store users and dashboards (and other
 things). By default it is configured to use `sqlite3` which is an
 embedded database (included in the main Grafana binary).
 
+### url
+Use either URL or or the other fields below to configure the database
+Example: `mysql://user:secret@host:port/database`
+
 ### type
 
 Either `mysql`, `postgres` or `sqlite3`, it's your choice.