Add instructions to migrate from node-red-contrib-ccu

[ptweety]

As this node provides the exact same nodes to Node-RED as node-red-contrib-ccu already does, both could not exist together in the same installation.

User need to know how they can safely migrate from one to the other.

In case users have this node installed in a dockerized Node-RED installation (not RedMatic) it may look like this:

docker cp <nodered-container>:/data/flows.json .
docker cp <nodered-container>:/data/flows_cred.json .

docker exec -it <nodered-container> /bin/bash
cd /data/

npm uninstall node-red-contrib-ccu
npm cache verify
npm install @ptweety/node-red-contrib-ccu

[ptweety]

Fixed by: https://github.com/ptweety/node-red-contrib-ccu/commit/35e926d1329567dfbdff0974dacdde85fda0c0a1