Define how prefixes work/are used

[paalbra]

After merging #49 the concept of prefixes are more complex. They're no longer just "text before some other text", but the PR includes the concept of a separator.

It's probably useful to say something about prefixes in the readme.

There should also be a single way to use a prefix. I.e. prefixes shouldn't both contain the separator some times and other times not contain the separator (this might not be a problem in the current code base).