docs: Update this super old readme (#36865)

This directory was created as part of an
idea [1] to create top-level openedx namespace which
every Open edX package would nest itself under.
The README was written with that in mind.

That never panned out, so openedx is really just
another "common" directory. Non-common code
should not be added to it. This update clarifies that.

[1] #5942 (comment)

Author: kdmccormick

openedx/README.rst

@@ -1,15 +1,8 @@
-Open edX
---------
+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.
+This directory (openedx) should contain code that is used by both `LMS <https://github.com/openedx/edx-platform/tree/master/lms>`_ and `CMS <https://github.com/openedx/edx-platform/tree/master/cms>`_. If your code is specific to LMS or CMS, put it in those directories instead.
 
-If you're adding a new Django app, place it in core/djangoapps. If you're adding
-utilities that require Django, place them in core/djangolib.  If you're adding
-code that defines no Django models or views of its own but is widely useful, put it
-in core/lib.
+Like openedx, the directory `common <https://github.com/openedx/edx-platform/tree/master/common>`_ also contains code used by both LMS and CMS. At some point we'll merge the two.
 
-Note: All new code should be created in this package, and the legacy code will
-be moved here gradually. For now the code is not structured like this, and hence
-legacy code will continue to live in a number of different packages.
+Lastly, the directory `xmodule <https://github.com/openedx/edx-platform/tree/master/xmodule>`_ contains legacy core code, also used by both LMS and CMS. We're in the middle of a long process of phasing that code out. Don't add new code there.