Clarify naming rules for custom licenses

[ryandesign]

The FAQ says:

https://github.com/fsfe/reuse-docs/blob/0913b0a83b36c161966be1c5e70c81bdadfb8a69/faq.md?plain=1#L544-L555

It is entirely unclear what is meant by "as such". Does it mean:

1. the file must be in the LICENSES directory
2. the file must be in the LICENSES directory and end with .txt
3. the file must be in the LICENSES directory and end with -MyLicense.txt
4. the file must be in the LICENSES directory and begin with LicenseRef- and end with .txt

I believe the answer is probably 4 but please specify it in the FAQ.

[silverhook]

The final result should be 4, yes.