Create readme for Flagd provider

[toddbaert]

Should include:

• basic overview
• link to flagd repo
• installation instructions (maven xml and gradle line)
• create issue in https://github.com/open-feature/docs.openfeature.dev/issues to add link to provider catalog.

[beeme1mr]

https://github.com/open-feature/java-sdk-contrib/tree/main/providers/flagd