Update exporters README (#210)
Update exporters README (#210)

Remove stale content, put in place a simple skeleton, and put information for the Zipkin exporter.
open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector