Merge pull request #20658 from edx/rir/readme

Add UML diagram for the ProgramEnrollments Plugin

lms/djangoapps/program_enrollments/readme.rst

+Program Enrollments
+======================
+
+This is the Program Enrollments plugin that allows university partners to
+enroll their students in Master's courses on the edx platform. 
+
+.. image:: doc_images/pe_uml.png
+
+Considerations:
+    - ProgramEnrollments
+        - Must have either user_id or external_user_key or both.
+        -  Valid values for the `status` field:
+            - `enrolled`
+            - `pending`
+            - `suspended`
+            - `withdrawn`
+    - ProgramCourseEnrollments
+        -  Valid values for the `status` field:
+            - `enrolled`
+            - `pending`
+            - `suspended`
+            - `withdrawn`
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