Add documentation about custom configuration for extensions

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