add some minimal installation instructions

lib/Components/ExtensionClass/README

-See the file ExtensionClass.stx or ExtensionClass.html for a
-description of extension class and for installation instructions. 
+See the file doc/ExtensionClass.stx or doc/ExtensionClass.html for a
+description of extension class.
+
+This version of ExtensionClass uses Python distutils for installation.
+It requires Python version 1.5.2 or higher.  To build, test, and
+install, do the following:
+
+% python setup.py build
+% PYTHONPATH=build/lib-PLAT-VER python test/regrtest.py
+% python setup.py install
+
+The build directory is created setup.py during the build.  The exact
+name of the build subdirectory depends on the platform you are using
+and the version of Python.
 
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