[Request]: README add note about VPN

[AVaccar]

Plugin

connectivity_plus

Use case

I spent a lot of time debugging why the application on my phone wasn't detecting connectivity changes when i was enabling/disabling the wifi.
In the end i had the duck duck go vpn enabled that was impeding network changes to be triggered.
The moment i disabled it, the Connectivity().onConnectivityChanged.listen started working as expected.

Proposal

Add a note in the readme about VPN next to the example of the stream subscription