docs: Add an OpenFeature feature matrix to the README

[kinyoklion]

Adds a feature matrix to the README mirroring the OpenFeature Ruby SDK's own feature table, so a reader can see at a glance what the provider does and does not support.

• One row per OpenFeature SDK feature (providers, targeting, hooks, logging, domains, eventing, shutdown, tracking, transaction context propagation, extending), plus initialization and flag metadata.
• Every row is currently supported following the tracking, lifecycle, eventing, wrapper, and flag metadata work; each note says how the provider implements it, or that the OpenFeature SDK provides it with no provider involvement.
• The legend keeps room for ⚠️/❌ so unsupported rows can state whether the limitation is in the OpenFeature Ruby SDK or is a provider gap.

Implementation details

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

Follow-up to the audit of LaunchDarkly's OpenFeature providers, which produced #27, #28, #29, #30 and #31 in this repository.

Describe the solution you've provided

The matrix reuses the row set and legend of the OpenFeature Ruby SDK feature table so the two can be read side by side, and so the same format can be applied to the other LaunchDarkly providers. Statements about eventing, initialization, shutdown, tracking, and flag metadata were checked against the current implementation in lib/ldclient-openfeature.

Describe alternatives you've considered

Writing a LaunchDarkly-specific row set instead of mirroring OpenFeature's — rejected, since the point of the matrix is comparability with the OpenFeature SDK table and with the other providers.

Testing

Documentation only; no code changes.

Additional context

Screenshots and a staging preview do not apply to a README change in a server-side library.

Link to Devin session: https://app.devin.ai/sessions/0c452d209ec54b068ba120b4c92b8f6c
Requested by: @kinyoklion

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Note

Overview
Adds an OpenFeature feature matrix to the README so readers can see at a glance what this provider supports.

The table mirrors the OpenFeature Ruby SDK feature list (providers, targeting, hooks, logging, domains, eventing, initialization, shutdown, tracking, transaction context, extending, flag metadata). Every row is marked supported, with notes on how the provider implements it or that the SDK handles it. The legend leaves room for partial/unsupported rows later.

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