Proper license template for Monero core and Monero GUI

[jermanuts]

Both Monero core and Monero GUI are using BSD-3-Clause license but it is not mentioned in LICENSE file.

Only monero-site seems to have proper template.

How to add template.

[selsta]

Seems to be a bug or issue with how Github detects the license?

[jermanuts]

@selsta this might be the issue?

https://stackoverflow.com/a/66297818 direct link to Github license detection docs.