Mention existing (insecure) Websockets implementations

[brownhoward]

Add a mention that existing Sold servers and clients implementing a notification protocol based on web sockets, and this being used for example for chat applications, collaborative editors, etc. It is disingenuous to write as though this is not currently already running. This protocol serves to provide a more secure and more modular version of the same functionality.

Originally posted by @timbl in solid/notifications-panel#3 (comment)