short explanation of new build requirements before docs are updated

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/trunk@193287 13f79535-47bb-0310-9956-ffa450edef68

README

@@ -13,4 +13,6 @@ FOP readme
 
     HTML-Documentation can be found in the subdirectory /docs. 
 
+    Compilation of Fop is started by executing build. It requires
+    Xerces-J, Xalan and bsf.jar (comes with Xalan) in your classpath.