added READMEs for top level directories

cms/README.rst

+CMS (Content Management System)
+-------------------------------
+
+This directory contains code relating to the course management portal for edX, also known as Studio.
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common/README.rst

+common
+------
+
+This directory contains common code shared between LMS and CMS, such as Mako templates, CSS, and Coffescript.
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lms/README.rst

+LMS (Learning Management System)
+--------------------------------
+
+This directory contains code relating to the student portal for edX.
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openedx/README.rst

+OpenEdx
+-------
+
+This is the root package for Open edX. 
+The intent is that all importable code from Open edX will eventually live here, including the code in the lms, cms, and common directories.
+
+Note: for now the code is not structured like this, and hence legacy code will continue to live in a number of different packages. 
+All new code should be created in this package, and the legacy code will be moved here gradually.
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