readme: Add deprecation notice

Add function to parse level from string without setting it

This can be used in combination with New(out, level).

Add readme

Write documentation

Add method to get the current log level.

Rename ParseLevel to SetLevelFromString

ParseLevel sounds more like it would return the level instead of setting
it.

Add simple Print wrapper functions

With that it is possible to directly log errors (like logger.Fatal(err)).

Add function to parse level from string

Initial commit