Document Resource Usage for setting span tags and service name

[ericmustin]

Following the work here: #8

We need to ensure that our documentation of best practice regarding resource usage is up to date. Specifically, we want users to rely on configuring a resource to define service name and arbitrary span tags, per:

open-telemetry/opentelemetry-ruby#312 (comment)

OpenTelemetry::SDK.configure do |c|
  c.resource = OpenTelemetry::SDK::Resources::Resource.create({
    'service_name' => 'my-application-name',
    'service.version' => version,
  })
end

Also should think about whether we can automate some of this so that setting environment variables can auto create resources. This would be similar to a resource detector

[ericmustin]

released in v0.2.0