Add what a BIP21 looks like to issue template

[Bosch-0]

In the below issue template, it should state what this looks like. This could simple state the generated address / QR has a bitcoin: parameter at the start.

[Bosch-0]

Also, they should generate not just a BIP21 QR code but an associated plaintext BIP21. I can't remember which but some wallets have the parameter in the QR but when copying to clipboard it is not present.

We may want to add some content around plaintext sharing of these unified payment requests also