Document OTEL_RESOURCE_ATTRIBUTES support in README

[trask]

Probably a new section, e.g. just above https://github.com/open-telemetry/opentelemetry-java-instrumentation#batch-span-processor.

Also, document how to use OTEL_RESOURCE_ATTRIBUTES to set the OTLP exporter service.name (probably directly within the OTLP config section, at least until #703 is resolved).

[tedsuo]

Just to clarify, does the OTEL_RESOURCE_ATTRIBUTES still exist? It seems like it is an option for OTLP, but also seems like it was removed in PR #424 (I assume it still does exist, based on the existence of this issue 😄 )

Related, there is an effort to standardize these common env vars here: open-telemetry/opentelemetry-specification#666

cc @anuraaga as I saw you use this config option.

[trask]

Hey @tedsuo, OTEL_RESOURCE_ATTRIBUTES wasn't removed in #424. That environment variable directly configures the OpenTelemetry Java SDK.

[tedsuo]

Derp, thank you!!