expound on java signer build instructions

external/source/msfJavaToolkit/compile.sh

@@ -1,5 +1,15 @@
 #!/bin/bash
 
+# This requires Java 1.7 or earlier because it uses private APIs.
+# See http://kris-sigur.blogspot.com/2014/10/heritrix-java-8-and-sunsecuritytoolskey.html
+# for more information.
+
+# Attempt to use Java 1.6 when building on OS X, otherwise JAVA_HOME needs to
+# be set manually.
+if [ -x /usr/libexec/java_home ]; then
+  export JAVA_HOME=$(/usr/libexec/java_home -v 1.6)
+fi
+
 javac -classpath $JAVA_HOME/lib/tools.jar:. javaCompile/*.java
 
 jar -cf msfJavaToolkit.jar javaCompile/*.class