Add Blackfire setup documentation

README.md

@@ -205,7 +205,27 @@ typing enter.
 
 ## Blackfire
 
-@todo
+In order to send profiles to Blackfire, you'll need to have a
+Blackfire agent reachable by the php-fpm image and the appropriate
+credentials.
+
+Providing an agent in docker compose is easy, as it's just starting
+the orignial Blackfire image.
+
+``` yaml
+  blackfire:
+    image: blackfire/blackfire
+    environment:
+      BLACKFIRE_SERVER_ID: <your server key>
+      BLACKFIRE_SERVER_TOKEN: <your server token>
+```
+
+Then you just need to add the `BLACKFIRE_CLIENT_ID` and
+`BLACKFIRE_CLIENT_TOKEN` to the `php-fpm` service definition.
+
+The correct id's and tokens can be found by viewing the [Blackfire
+setup documentation](https://blackfire.io/docs/php/configuration) when
+logged in.
 
 ## Mail