Expain when `gradle run` is ready

It isn't obvious.

Closes #57097

CONTRIBUTING.md

@@ -115,10 +115,20 @@ onwards and Eclipse 2020-3 and onwards.
 
     ./gradlew :run
 
-You can access Elasticsearch with:
+That'll spend a while building Elasticsearch and then it'll start Elasticsearch,
+writing its log above Gradle's status message. We log a lot of stuff on startup,
+specifically these lines tell you that Elasticsearch is ready:
+
+    [2020-05-29T14:50:35,167][INFO ][o.e.h.AbstractHttpServerTransport] [runTask-0] publish_address {127.0.0.1:9200}, bound_addresses {[::1]:9200}, {127.0.0.1:9200}
+    [2020-05-29T14:50:35,169][INFO ][o.e.n.Node               ] [runTask-0] started
+
+But to be honest its typically easier to wait until the console stopps scrolling
+and then run `curl` in another window like this:
 
     curl -u elastic:password localhost:9200
 
+
+
 ### Importing the project into IntelliJ IDEA
 
 Elasticsearch builds using Java 14. When importing into IntelliJ you will need