Clarify docs for custom logger backends

[fajpunk]

The docs currently make it seem (to me, at least) that wrap-with-logger accepts a single map argument to define custom loggers, when in fact, it accepts a variable number pairs of level keywords and logger functions.

Here's an attempt to update the Readme to make this more clear, if you like it :)

[pjlegato]

Thanks, that's definitely an improvement.