[doc][java] Add hint for Guava users in InefficientEmptyStringCheck

Fixes issue #571.

pmd-java/src/main/resources/rulesets/java/strings.xml

@@ -221,8 +221,9 @@ if (s.indexOf('d') {}
 String.trim().length() is an inefficient way to check if a String is really empty, as it
 creates a new String object just to check its size. Consider creating a static function that
 loops through a string, checking Character.isWhitespace() on each character and returning
-false if a non-whitespace character is found. You can refer to Apache's StringUtils#isBlank (in commons-lang)
-or Spring's StringUtils#hasText (in the Springs framework) for existing implementations.
+false if a non-whitespace character is found. You can refer to Apache's StringUtils#isBlank (in commons-lang),
+Spring's StringUtils#hasText (in the Spring framework) or Google's CharMatcher#whitespace (in Guava) for
+existing implementations.
     </description>
     <priority>3</priority>
     <example>