Update README.md Unifi Plugin

[plasmediaX]

already prepared an update for the Unifi plugin containg the following changes:

• difference between Hardware and software-based controllers
• description of the port field and its different values
• made clear that only local users can login

please wait with publishing until the latest plugin with the described changes is in experimental and I could test it.

Thanks.

nymea-plugins pull request checklist:

• [ ] Make sure the pull request's title is of format "Plugin name: Add support for xyz" or "New plugin: Plugin name"

• [ ] Did you test the changes on hardware, if not (e.g. absence of required hardware), please mention a person to confirm it has been tested.

• [ ] Did you update the plugin's README.md accordingly?

• [ ] Did you update translations (cd builddir && make lupdate)?