Updated readme for building documentation.

README.md

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+=========================================
+glymur: a Python interface for JPEG 2000
+=========================================
+
+**glymur** contains a Python interface to the OpenJPEG library
+which allows linux and mac users to read and write JPEG 2000 files.  
+**glymur** works on Python 2.7 and 3.3.  Python 3.3 is strongly recommended.
+
+Until I figure out how to get readthedocs.org to handle numpydoc, you'll have
+to do as follows to build the documentation first::
+    
+    $ make -C docs html
+    $ open docs/build/html/index.html