Add to readme

Author: FrankSalad

docs/api/database.md

@@ -25,6 +25,17 @@ firestack.database()
   });
 ```
 
+Basic write with priority example:
+```javascript
+firestack.database()
+  .ref('posts/1235')
+  .setWithPriority({
+    title: 'Another Awesome Post',
+    content: 'Some awesome content',
+  }, 10);
+```
+Useful for `orderByPriority` queries.
+
 ## Unmounted components
 
 Listening to database updates on unmounted components will trigger a warning: