Docs: Add an example to CONTRIBUTING.md (elastic#50398)

Adds an example of accessing Elasticsearch after building it from source
for the first time to CONTRIBUTING.md. I'm sure how to do it is
intuitive to some folks but I had to read three two build.gradle file to
find the password and I'd like to save other new folks the trouble.

CONTRIBUTING.md

@@ -119,6 +119,10 @@ For IntelliJ, the minimum version that we support is [IntelliJ 2017.2][intellij]
 
     ./gradlew :run
 
+You can access Elasticsearch with:
+
+    curl -u elastic:password localhost:9200
+
 ### Configuring IDEs And Running Tests
 
 Eclipse users can automatically configure their IDE: `./gradlew eclipse`