add nginx selinux to docs (#5710)

Co-authored-by: Simon Smith <simonsmith5521@gmail.com>

docs/docs/meshcentral/index.md

@@ -827,6 +827,12 @@ In this example, we will:
 - MeshCentral will read the NGINX web certificate so agents will perform correct server authentication.
 - NGINX will be setup with long timeouts, because agents have long standard web socket connections.
 
+!!!note
+  With SELinux, NGINX reverse proxy requires 'setsebool -P httpd_can_network_relay 1'
+  Caution: httpd_can_network_relay only allows certain ports
+           Confirm you are using ports from this subset in MeshCentral
+           If you want to use a different port then you will need to add it to http_port_t
+
 Let’s get started by configuring MeshCentral with the following values in config.json:
 
 ```json