Add note about HMR WebSocket to upgrade guide (#30704)
Add note about HMR WebSocket to upgrade guide (#30704)

Co-authored-by: Lee Robinson <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@leerob.io>
JeromeFitz/next.js,azukaru/next.js,azukaru/next.js,zeit/next.js,azukaru/next.js,azukaru/next.js,JeromeFitz/next.js,JeromeFitz/next.js,zeit/next.js,JeromeFitz/next.js,zeit/next.js