docs: Add an OpenFeature feature matrix to the README

[kinyoklion]

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

• Most unsupported rows are limitations of the OpenFeature PHP SDK, not of this provider — eventing, shutdown, named clients, tracking, transaction context and flag metadata have no place in that SDK's provider interface — and each row says so, so they are not read as provider gaps.
• Format matches the matrices added to the Java, Python, .NET and JavaScript providers this week, so the providers can be compared side by side.

Implementation details

Rows and statuses were checked against the OpenFeature PHP SDK 2.3.0 and the LaunchDarkly PHP SDK 6.8.3 rather than copied from another provider: OpenFeature\interfaces\provider\Provider declares only the five resolve*Value methods plus hooks, logger and metadata accessors, ProviderAware holds a single provider for the whole API, and neither EvaluationDetails nor ResolutionDetails carries flag metadata.

The three separate fixes opened alongside this (#27, #28, #29) are provider-side bugs within rows the matrix marks as supported, so they do not change any status here.

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

None.

Describe the solution you've provided

A ## Feature matrix section after the supported-versions section, with the same Status | Feature | Notes shape as the other LaunchDarkly OpenFeature providers.

Describe alternatives you've considered

Listing only the rows the OpenFeature PHP SDK's own matrix has: it would drop initialization, tracking, transaction context and flag metadata, which are exactly the rows readers coming from another LaunchDarkly provider will look for.

Additional context

Testing: documentation only; make check still passes.

Link to Devin session: https://app.devin.ai/sessions/38a6eaf69fcf41109e136a1d0fe5e899
Open in Devin Desktop: https://app.devin.ai/desktop/session/38a6eaf69fcf41109e136a1d0fe5e899?variant=devin
Requested by: @kinyoklion

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Note

Overview
Adds a Feature matrix section to the README (after supported PHP versions), aligned with the OpenFeature PHP SDK feature list and the same table shape used on other LaunchDarkly OpenFeature providers.

The table lists each capability with ✅/❌ status and notes: supported areas (flag types, targeting, hooks, logging, MultiProvider, extending via LDClient) and unsupported rows that call out whether the gap is the OpenFeature PHP SDK (e.g. named clients, eventing, shutdown, tracking, transaction context, flag metadata) versus provider behavior.

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