Add project models ERD with descriptions

Models.md

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+# Entity-relationship diagram
+```mermaid
+erDiagram
+USER }o--o{ USER : contact
+CONFIG ||--|| USER : has
+MESSAGE }o--|| USER : write
+MESSAGE }o--o{ MESSAGE : "answer to"
+GROUP }o--o{ USER : joined
+```
+
+# Models
+`USER`: Represents individual users.  
+`CONFIG`: Represents configurations or settings.  
+`MESSAGE`: Represents individual messages sent or received.  
+`GROUP`: Represents a group of users.
+
+# Relations
+
+## USER with USER
+Represents a many-to-many relationship between users, indicating that users can be in contact with multiple other users and vice versa. This could represent a social network-like relationship where users can communicate with each other.
+
+## USER with CONFIG
+This notation signifies a one-to-one relationship, indicating that each user has one specific configuration, and that configuration is owned by that user exclusively. This relationship implies a direct mapping between a user and their unique configuration settings within the system.
+
+## USER with MESSAGE
+Represents a one-to-many relationship between USER and MESSAGE, indicating that a user can write multiple messages, but each message is written by only one user.
+
+## MESSAGE with MESSAGE
+Represents a many-to-many relationship between messages, indicating that a message can be an answer to multiple other messages, and vice versa. This could represent threaded or nested messages where one message is a reply to another.
+
+## GROUP with USER
+Represents a many-to-many relationship between GROUP and USER, indicating that users can join multiple groups, and a group can have multiple users as members.