Document how to change logging configuration

There's a file config/log4j2.xml. It's a standard Log4j2 config file. Log4j2 reference is here: https://logging.apache.org/log4j/2.x/manual/configuration.html

 
Bits to change would be the Root logging level – change <Root level="info"> to <Root level="debug">
 
and maybe also <Logger name="com.couchbase.client.core" level="warn"/> to <Logger name="com.couchbase.client.core" level="debug"/>