Updates README.md with monolog handler mention.

Author: phpeek

README.md

@@ -90,6 +90,20 @@ sentry:
     dsn: "https://public:[email protected]/1"
 ```
 
+#### Optional: use monolog handler provided by `sentry/sentry`
+*Note: this step is optional*
+
+The bundle registers `Sentry\Monolog\Handler` as a service. If You're using `monolog` for logging e.g. in-app errors, You
+can use this handler in order for them to show up in Sentry. Modify `MonologBundle` configuration: 
+
+```yaml
+monolog:
+    sentry:
+      type: service
+      id: Sentry\Monolog\Handler
+      level: error 
+```
+
 ## Maintained versions
  * 3.x is actively maintained and developed on the master branch, and uses Sentry SDK 2.0;
  * 2.x is supported only for fixes; from this version onwards it requires Symfony 3+ and PHP 7.1+;