Create doc for custom User model

docs/decisions/0003-custom-user-model.md

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+# 0002 Use Custom User Model
+
+- Date: 2024-04-10
+- Author(s): [Chen Zhang][chen]
+- Status: `Active`
+
+## Decision
+
+Instead of using Django built-in User model, We will use a custom User model which
+is based on the Django built-in AbstractUser model and provides all the same features
+by default.
+
+## Context
+
+Django comes with a built-in User model which provides basic authentication and 
+authorisation features. This model includes fields like username, email, password,
+and essential methods for user management. However, there're some limitations of
+the default User model.
+
+The built-in User model is designed so users can login via username and password,
+and won’t be able to login with email and password. Also, we're unable to add new
+fields to the default user model.
+
+[The official Django documentation][document] highly recommends using a custom user
+model when start a Django project, because it’s hard to switch a user model out for
+another once the project is deployed and being used by real users.
+
+## Implications
+
+This custom User model might not be perfect for all use cases,  but by making a custom
+user model part of the template project we can insure it is as easy as possible to
+change and evolve that model later one. Something this is a lot more difficult should
+we stick with the built in one.
+
+In some cases, users might lost access to their email address before they update their
+email address on the website or APP, and if the project uses email as primary user identifier
+it will be difficult for them to get their accounts back.
+
+<!-- Links -->
+[chen]: mailto:[email protected]
+[document]: https://docs.djangoproject.com/en/5.0/topics/auth/customizing/#using-a-custom-user-model-when-starting-a-project