Update README with more component details

[janfockaert]

All components in the project are conditional and can be overridden. Add the following topics:

• Give an overview of all components.
• Provide an example on how to override such a component.

[MartinX3]

And update the Example Readme.
There is many stuff missing, I'm building on my own.
Like adding a field with subfields.

[janfockaert]

For specific readme about adding fields, ... I would refer to GraphQl Java library instead. For this readme the scope is about "Spring Boot Starter GraphQl" project.