Add note on testing multicast

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1488139 13f79535-47bb-0310-9956-ffa450edef68

BUILDING.txt

@@ -293,7 +293,6 @@ directory:
 
     output/build/logs
 
-
 By default the testsuite is run three times to test 3 different
 implementations of Tomcat connectors: BIO, NIO and APR. (If you are not
 familiar with Tomcat connectors, see config/http.html in documentation for
@@ -318,6 +317,12 @@ If you are on Windows and want to test the APR connector you can put the
 tcnative-1.dll file into ${tomcat.source}/bin/native/ and it will be copied
 into the above directory when the build runs.
 
+The unit tests include tests of the clustering functionality which require
+multicast to be enabled. There is a simple application provided in the Tomcat
+test source (org.apache.catalina.tribes.TesterMulticast) that can be used to
+check if a machine supports multicast. Notes on enabling multicast for different
+operating systems are provided in the Javadoc for that class.
+
 
 (7.2) Running a single test