Specify disabling options in README

README.md

@@ -120,6 +120,10 @@ The trace agent can be configured by passing a configurations object to the agen
 
 One configuration option of note is `enhancedDatabaseReporting`. Setting this option to `true` will cause database operations for redis and MongoDB to record query summaries and results as labels on reported trace spans.
 
+## Disabling the trace agent
+
+The trace agent can be turned off by either setting the `GCLOUD_TRACE_DISABLE` environment variable or specifying `enabled: false` in your configuration file.
+
 ## Trace batching and sampling
 
 The aggregation of trace spans before publishing can be configured using the `flushDelaySeconds` and `bufferSize` [options](config.js). The spans recorded for each incoming requests are placed in a buffer after the request has completed. Spans will be published to the UI in batch when the spans from `bufferSize` requests have been queued in the buffer or after `flushDelaySeconds` have passed since the last publish, whichever comes first.