Documentation about running one unit test

It is written here
https://github.com/groovy/groovy-core/blob/master/gradle/test.gradle#L66
but I think it is worth writing it on README.adoc.

Author: yukoba

README.adoc

@@ -49,6 +49,10 @@ To build everything and launch unit tests, use
 
     ./gradlew test
 
+If you want to launch one unit test, use this. <TestClassName> is like `groovy.GroovyMethodsTest`.
+
+    ./gradlew :test --tests <TestClassName>
+
 To build from IntelliJ IDEA
 
     ./gradlew jarAll idea