Document out-of-source build.

doc/INSTALL.txt

@@ -74,7 +74,7 @@ Quick Start
 
 Building Zope
 
-  If you want to try out Zope in the simplest fashion, then run the
+  If you want to try out Zope in a more permanent fashion, then run the
   conventional UNIX build sequence::
 
     ./configure --prefix /where/to/install/zope
@@ -114,6 +114,13 @@ Building Zope
 
     * See CHANGES.txt for important notes on this version of Zope.
 
+    * You can run the 'configure' command outside of the Zope source
+      tree.  If you do so, the makefile and the files generated by the
+      build process will be written to the directory from which you
+      run 'configure'.  You can then use 'make install' to install these
+      files to their canonical locations.  This is to support
+      source locations which are not writable.
+
 Integrating Zope with an existing webserver
 
   Zope doesn't require any existing webserver to run, but you can