Enhance README with Detailed Introduction of GFlowNet Components

[engmubarak48]

The current README provides a general overview of the GFlowNet library. To help new users better understand the utility and structure of our library, we need to:

• Expanding the README to include detailed descriptions of the main components of GFlowNet library.
• Include clear explanations of key modules, their functionalities, and how they interact within the framework.
• Adding diagrams and simple use-case examples