Clarify naming rules for custom licenses

[Ryan Carsten Schmidt]

The FAQ says:

reuse-docs/faq.md

Lines 544 to 555 in 0913b0a

 If you have a custom or modified license that does not appear in the SPDX 
 License List, place your license in the file 
 `LICENSES/LicenseRef-MyLicense.txt`. By naming your license as such, tools that 
 speak SPDX will still be able to recognise your license. 
  
 In this example, the header in files covered by this custom license may look 
 like the following: 
  
 ``` 
 # SPDX-License-Identifier: LicenseRef-MyLicense 
 # SPDX-FileCopyrightText: 2017 Jane Doe <[e-mail]> 
 ``` 

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.

[Matija Šuklje]

The final result should be 4, yes.