UPdate README

git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@11134 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689

README

@@ -7,6 +7,10 @@ the required software.  If you have already completed the steps outlined in the
 INSTALL file, then continue reading for examples how to convert these files into
 various other formats.
 
+In all examples, setting the parameter REV=systemd is needed to build the 
+systemd version of the book.
+
+
 XML to XHTML:
 -------------
 make BASEDIR=/path/to/output/location