I spent around 50minutes debugging and reading the source codes to find out
that you the reuse tool expects texts of licenses in the directory LICENSES.

This commit adds a hint message for slower people like me.

From:
```
 # MISSING LICENSES

'CC-BY-4.0' found in:
* CONTRIBUTING.md
* README.md

 # SUMMARY

* Bad licenses: 0
* Deprecated licenses: 0
* Licenses without file extension: 0
* Missing licenses: CC-BY-4.0
* Unused licenses: 0
* Used licenses: CC-BY-4.0
* Read errors: 0
* files with copyright information: 2 / 2
* files with license information: 2 / 2

Unfortunately, your project is not compliant with version 3.0 of the REUSE Specification :-(
```

To:
```
 # MISSING LICENSES
The corresponding license txt files were not found in the directory LICENSES

'CC-BY-4.0' found in:
* CONTRIBUTING.md
* README.md

 # SUMMARY

* Bad licenses: 0
* Deprecated licenses: 0
* Licenses without file extension: 0
* Missing licenses: CC-BY-4.0
* Unused licenses: 0
* Used licenses: CC-BY-4.0
* Read errors: 0
* files with copyright information: 2 / 2
* files with license information: 2 / 2

Unfortunately, your project is not compliant with version 3.0 of the REUSE Specification :-(
```