add new env var to README

Author: stefan.rieckhof

maintenance/tracing/README.md

@@ -8,4 +8,5 @@ Property| Description
 --- | ---
 `SENTRY_DSN`  | The DSN to use.
 `ENVIRONMENT` | The environment to be sent with events.
-`SENTRY_TRACES_SAMPLE_RATE` | The tracing sample rate to use (default 0.1).
+`SENTRY_TRACES_SAMPLE_RATE` | The tracing sample rate to use (default: 0.1).
+`SENTRY_ENABLE_TRACING` | Enable or disable tracing (default: true).