Clearly document the logging requirements for the DI framework

https://github.com/brantburnett/Couchbase.Extensions/issues/16

Specifically, we need to document that some implementation of Microsoft.Extensions.Logging.ILoggerFactory must be registered side-by-side with the dependency injection system. If running an ASP.NET Core app, this should be done already for free. However, a console app may not have this in place so it must be manually included when building the DI container.

Many different logging frameworks could be used, so long as it's compatible with Microsoft.Extensions.Logging.Abstractions. This includes Microsoft's logging framework, and others such as Serilog.