Merge pull request #20345 from edx/feanil/user_api_readme

Readme for the user_api.

openedx/core/djangoapps/user_api/README.rst

+Status: Active
+
+Responsibilities
+================
+The user_api app is currently a catch all that is used to provide various apis that are related to the user and also to features within the platform.
+
+Intended responsibility: To manage user profile and general account information and to provide APIs to do so easily.
+
+Direction: Decompose
+===============
+Currently this app is a catch all for many user related information even when that information sholud really belong in a different app.  If you are building a feature and need to provide information about a user within the context of your feature, you should localize that API to your feature and make your assumptions about what user information you need clear.
+
+For example authentication related APIs have already been moved to the user_authn django app.
+
+Glossary
+========
+
+More Documentation
+==================