Add documentation on user permissions

docs/admin/permissions.md

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+---
+title: User Permissions
+---
+
+## Permissions
+
+!!! warning "TODO"
+    This section requires further work - in particular screenshots!
+
+InvenTree provides a permission system which allows authority control on a user or group basis.
+
+!!! info "Django Permissions"
+    InvenTree implements the django permissions system. For background reading, refer to the [django permissions documentation](https://docs.djangoproject.com/en/3.1/topics/auth/default/#permissions-and-authorization).
+
+Permissions can be assigned for each model in the InvenTree database. The following permission classes are available for each model (or "table") in the database:
+
+### View
+
+Users with the *View* permission for a particular model will be able to view data associated with the model.
+
+*For example, a user who has the View permission set for the PurchaseOrder model will be able to view purchase orders.*
+
+### Add
+
+Users with the *Add* permission for a particular model will be able to add (create) new instances of that model.
+
+*For example, a user who has the Add permission set for the StockItem model will be able to add / create new stock item objects.*
+
+### Edit
+
+Users with the *Edit* permissions for a particular model will be able to edit (adjust) instances of that model.
+
+*For example, a user who has the Edit permission set for the Build model will be able to edit Build items.*
+
+### Delete
+
+Users with the *Delete* permission for a particular model will be able to delete instances of that model.
+
+## Superuser Account
+
+The *superuser* account (normally the first user created when configuring the database) automatically has every assigned permission.
+
+## Assigning Permissions
+
+The *superuser* account can assign model permissions to any users or groups.
+
+Additionally, any users who have permissions to edit the *Users* table can also adjust these permissions.
+
+### User Permissions
+
+User permissions allow model permissions to be assigned on a single user basis. This is useful if you wish to finely control which InvenTree features a certain user can access.
+
+### Group Permissions
+
+Group permissions allow model permissions to be assigned to a *group* of users which greatly simplifies the task of assigning similar permissions to multiple users.
+
+## Admin Interface Permissions
+
+If a user does not have the required permissions to perform a certain action in the admin interface, those options not be displayed.
+
+If a user is expecting a certain option to be available in the admin interface, but it is not present, it is most likely the case that the user does not have those permissions assigned. 
+
+## Web Interface Permissions
+
+When using the InvenTree web interface, certain functions may not be available for a given user, depending on their permissions. In this case, user-interface elements may be disabled, or may be removed.
+
+## API Permissions
+
+When using the InvenTree API, certain endpoints or actions may be inaccessible for a given user, depending on their permissions.
+
+As the API is used extensively within the web interface, this means that many data tables may also be impacted by user permissions.