Add readme for flagd provider

[agardnerIT]

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

It would be great to have a readme with some sample snippets of how to use the flagd Python provider.

@tcarrio tagging you as the last person to touch the flagd provider (and thus probably most likely to be the expert).

[federicobond]

We just released openfeature-provider-flagd with a relatively spartan but usable README.