Add custom licence identifier usage to FAQ answer

[uiopaubo]

In the FAQ answer providing instructions on employing a custom licence...

https://reuse.software/faq/#custom-license

...it might be worthwhile to provide an example that uses the custom licence identifier, just to remove any doubt about the exact form it takes. The example licence filename...

LICENSES/LicenseRef-MyLicense.txt

...corresponds to the identifier LicenseRef-MyLicense. Given the usual translation of identifiers to filenames, one might have expected the identifier to be this (merely removing the extension), but as the edit history to this comment can attest, it is easy to think that the identifier might just be MyLicense. So there is potential for some confusion.

[Matija Šuklje]

It does correspond to the SPDX Identifier LicenseRef-MyLicense.

To clarify, the header:

# SPDX-FileCopyrightText: 2020 Doood <[e-mail]>
#
# SPDX-License-Identifier: LicenseRef-DooodsLicense

means the license is to be found in LICENSES/LicenseRef-DooodsLicense.txt

I agree that it might be useful to state that explicitly in the FAQ.