Add logging behaviour description to README.md

Give an example of how to turn logging on, and also how to enable debug mode.

I've put this PR in after @PeterNachbaur and @wetzler helped me work out via email why Exceptions weren't being thrown. I thought it might be useful to anyone else who has the same problem.
FYI, in addition to the "dummy API key" scenario you mentioned, there is a second reason for catching and ignoring exceptions: we never want to accidentally cause an application to crash if we throw an unchecked exception. This description is fine for the README though :)