Add comprehensive and consistent readme

[toddbaert]

Update readme to be consistent with the template here: https://github.com/open-feature/.github/tree/main/templates/READMEs

Definition of done:

• README structure matches template
• Examples are up-to-date and added for all TODOs in template
• details only pertaining to developers of the SDK are moved to CONTRIBUTING.md

See similar Java PR: open-feature/java-sdk#382