Added class attribute description to readme.tex

xml/readme.tex

diff --git a/xml/readme.tex b/xml/readme.tex
index 36a24f5..f39558a 100644
--- a/xml/readme.tex
+++ b/xml/readme.tex
@@ -539,6 +539,9 @@
       string expression with poorly defined syntax, usually representing
       a length that is computed as a combination of other command
       parameter values, and possibly current GL state as well.
+\item \attr{class} - an arbitrary string, defining type of object name (object handle).
+      Handles with different types are not interchangeable. So handle created as
+      \attr{class=buffer} can not be passed to \attr{class=shader}.
 \end{itemize}
 
 If the group name is defined, it may be interpreted as described in