docs: update project readme

[Regaez]

The readme file is currently the default template for a Grav plugin. It needs to be updated with more relevant information.

• Configuration and setup of plugin
• Granting user permissions for API usage
• Roadmap/ desired features
• More details regarding development environment
• Warning about using API on your site. Potential security vulnerability etc.
• Supported version of Grav
• ... etc.

[Regaez]

• Remove the "installation by gpm" section until the plugin is actually released to gpm.
• add a section "when to use?" comparing the api plugin vs grav's existing content type functionality. You might not need the api.