Document fast profile in contributing documentation

Closes gh-8424

Author: snicoll

CONTRIBUTING.adoc

@@ -88,12 +88,12 @@ The project can be built from the root directory using the standard maven comman
 NOTE: You may need to increase the amount of memory available to Maven by setting
 a `MAVEN_OPTS` environment variable with the value `-Xmx512m`
 
-If you are rebuilding often, you might also want to skip the tests until you are ready
-to submit a pull request:
+If you are rebuilding often, you might also want to skip the tests and the execution of
+checkstyle until you are ready to submit a pull request:
 
 [indent=0]
 ----
-	$ ./mvnw clean install -DskipTests
+	$ ./mvnw clean install -DskipTests -Pfast
 ----