Updating readme with RBAC

[ankush-cohere]

This PR introduces a new section on Role-Based Access Control (RBAC) in the README.md file, providing a code snippet for setting up user access control using the Cohere Compass API. The code demonstrates how to create users, add them to groups, create roles with specific permissions, and map roles to groups.

Changes:

• Added a new section titled "RBAC" to the README.md file.
• Introduced Python code for user access control using the Cohere Compass API.
• Imported necessary modules from cohere.compass.clients.rbac, cohere.compass.models.rbac, and requests.exceptions.
• Defined variables for API configuration, user names, group name, role name, indexes, and permissions.
• Created users and added them to a group using the CompassRootClient instance.
• Created a role with specific permissions and mapped it to the group.
• Handled potential HTTP errors during the user creation process.