Update README file

[Sheila-nk]

Proposed change

• Refine description of the JupyterHub Pytest Plugin project
• Add a Usage section for plugin installation and fixture importation
• Provide an example of plugin integration using the DockerSpawner draft PR to demonstrate usage
• Add workflow status badges

Who would use this feature?

This feature is useful for users of and contributors to the JupyterHub Pytest Plugin