Update the SDK to be compliant with the v0.5.1 spec

[toddbaert]

Update the SDK to be compliant with the v0.5.1 spec.

What's Changed

• chore: add additional document statuses by @toddbaert in chore: add additional document statuses spec#139
• chore: add status badges by @toddbaert in chore: add status badges spec#148
• chore: section clarity, harden API, providers by @toddbaert in chore: section clarity, harden API, providers spec#149

Update the badge on the readme to

Full Changelog: open-feature/spec@v0.5.0...v0.5.1

[toddbaert]

There's probably nothing to do here other than update some tests. I will verify.