Add new pre-requisite to PRO deployments

docs/deployment/pro/on-premises.md

@@ -91,6 +91,8 @@ Once you have your instances ready, be sure to meet the following criteria in th
 
 - **Configure a domain name**: OpenVidu Pro is deployed using HTTPS because it is mandatory to use WebRTC. Then, if you do not have a domain name, an autogenerated SSL certificate will be used and an ugly warning will appear to your users when entering to your site. And of course you can suffer a man-in-the-middle attack. So it is recommended that you configure a domain name pointing to Master Node public IP (Media Nodes do not need a domain name, it is enough for them to have a public IP). A valid SSL certificate can be automatically generated using Let's Encrypt in the installation process. If you already have a valid SSL certificate of your own, it also can be configured.
 
+- **Allow access to endpoint `https://accounts.openvidu.io`**: it is mandatory that your Master Node is able to reach this endpoint. It is used to check the license key and register its usage. If you are deploying OpenVidu Enterprise with [mediasoup](openvidu-enterprise/#kurento-vs-mediasoup), then it is also mandatory that your Media Nodes have also access to that endpoint.
+
 - **Port configuration in _Master Node_**
 
     - **Open these ports** ([here](#close-ports-to-avoid-external-attacks) you have an UFW sample to configure a firewall)