added warning (Python2.1 and -lpth)

doc/INSTALL.txt

@@ -71,11 +71,13 @@ Building Zope
 
   Notes
   
-    * You should be using Python 1.5.2 to run the build scripts.
+    * You should be using Python 2.1 to run the build scripts.
 
     * The python you run Zope with *must* have threads compiled in,
       which is *not* the case for a vanilla build.  When you build the
       python you'll use, 'configure --with-threads' does the right thing.
+      Warning: Zope will not run with a Python version that uses libpth.
+      You *must* use libpthread.
 
     * To build python extensions you need to have Python configuration
       information available. If your Python comes from an RPM you may need