update Readme.md with an note where to get private key

[novski]

Hi
I had a hard time figuring out where to find the right information to connect with a Laptop and WireGuard app.
The wrong thought that I made was that I tried to use the shown information in your wonderful wizard.
But the config of the app needs not the public key but the private key and calculates the private key by itself.
Here is a screenshot:

After several attempts I found an issue explaining where to find the private key and using the private key was successful.
My recommendation would be to either display the private key in the wizard or update the documentation (Readme) with a line where to find it.
It's available in the webconfig of the ER under: Device Tree interface/wireguard/wg0/

[vchrizz]

I guess this is related to #14