Add 'w3c' to valid JAEGER_PROPAGATION options (#736)

Signed-off-by: chgl <[email protected]>

jaeger-core/README.md

@@ -96,7 +96,7 @@ JAEGER_ENDPOINT | no | The traces endpoint, in case the client should connect di
 JAEGER_AUTH_TOKEN | no | Authentication Token to send as "Bearer" to the endpoint
 JAEGER_USER | no | Username to send as part of "Basic" authentication to the endpoint
 JAEGER_PASSWORD | no | Password to send as part of "Basic" authentication to the endpoint
-JAEGER_PROPAGATION | no | Comma separated list of formats to use for propagating the trace context. Defaults to the standard Jaeger format. Valid values are **jaeger** and **b3**
+JAEGER_PROPAGATION | no | Comma separated list of formats to use for propagating the trace context. Defaults to the standard Jaeger format. Valid values are **jaeger**, **b3**, and **w3c**
 JAEGER_REPORTER_LOG_SPANS | no | Whether the reporter should also log the spans
 JAEGER_REPORTER_MAX_QUEUE_SIZE | no | The reporter's maximum queue size
 JAEGER_REPORTER_FLUSH_INTERVAL | no | The reporter's flush interval (ms)