Improve informative message when .license file has been created

[mxmehl]

Source: #706

If you run the annotate command, you will be informed about that a .license file has been created:

$ reuse annotate -c me -l MIT --style=html test.svg
Successfully changed header of test.svg.license

It's not entirely obvious that a separate file has been created, even if a specific style has been chosen.

@niccokunzmann proposed the message Successfully changed header of test.svg.license because test.svg must not contain a REUSE header.

• Whenever you do not use --force-dot-format, this does not need to appear.
• I do not think that this needs to appear when the .license is used automatically
• But if I choose a style and you choose not apply it. I could value an error + message or the message above.