Document diagnostic logging with environment variable

[dyladan]

Create a troubleshooting section on the readme which is a little more prominent

[jtmalinowski]

I could improve this a little bit, if you think it's still needed. What do you think is missing in this https://github.com/open-telemetry/opentelemetry-js#setting-the-global-diagnostic-logger.

The title also suggests that you could enable logging with an environment variable. Isn't the default logger noop?

[dyladan]

The linked docs are in the main repo where this package is moved to and there is an issue to enable the environment variable. I'm closing this issue