Docs: add security section (#416)

* Docs: add security section

* Apply review feedback

Author: AgnesToulet

README.md

@@ -108,6 +108,12 @@ The following example describes how to build and run the remote HTTP rendering s
 
 1. Restart Grafana.
 
+## Security
+
+Access to the rendering endpoints is restricted to requests providing an auth token. This token should be configured in the Grafana configuration file and the renderer configuration file. This token is important when you run the plugin in remote rendering mode to avoid unauthorized file disclosure (see [CVE-2022-31176](https://github.com/grafana/grafana-image-renderer/security/advisories/GHSA-2cfh-233g-m4c5)).
+
+See [Grafana Image Rendering documentation](https://grafana.com/docs/grafana/latest/image-rendering/#security) to configure this secret token. The default value `-` is configured on both Grafana and the image renderer when you get started but we strongly recommend you to update this to a more secure value.
+
 ## Configuration
 
 For available configuration settings, please refer to [Grafana Image Rendering documentation](https://grafana.com/docs/grafana/latest/image-rendering/#configuration).