Add documentation about custom configuration for extensions

[patrickelectric]

at the moment you can set a custom configuration in the extension.
To do that, go in the extensions page -> installed -> Go to the extension block and click in Edit.
There you should add the following json configuration:

{
  "ExposedPorts": {
    "80/tcp": {}
  },
  "HostConfig": {
    "ExtraHosts": [
      "blueos.local:host-gateway"
    ],
    "Privileged": true,
    "Binds": [
      "/usr/blueos/extensions/node-red:/data:rw",
      "/etc/hostname:/etc/hostname:ro",
      "/dev:/dev:rw",
      "/:/home/workspace/host:rw"
    ],
    "PortBindings": {
      "80/tcp": [
        {
          "HostPort": "4242"
        }
      ]
    }
  }
}

after that, the port 4242 will be used in this example