docs: Add an OpenFeature feature matrix to the README

[kinyoklion]

Adds a feature matrix to the README mirroring the OpenFeature .NET SDK's own feature table, so a reader can see at a glance what this provider supports.

• Same rows and order as the OpenFeature .NET SDK matrix, plus Initialization and Flag metadata
• Rows that are not fully supported state whether the limitation is in the OpenFeature SDK or in the provider, and link the tracking issue
• Only DependencyInjection (experimental) is marked partial, pointing at #48 and #49; everything else is supported
• The same matrix format is being added to the Java, Python and JavaScript providers, and already exists in the Ruby provider, so they can be compared side by side

Implementation details

Written during the weekly OpenFeature provider audit. Each row was checked against the provider source rather than assumed; the wording follows the matrix already present in the Ruby provider README.

Requirements

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

Related issues

#48

Link to Devin session: https://app.devin.ai/sessions/38a6eaf69fcf41109e136a1d0fe5e899
Requested by: @kinyoklion

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Note

Overview
Adds a Feature matrix section to the README (between supported .NET versions and Getting started) that aligns with the OpenFeature .NET SDK feature table, plus rows for Initialization and Flag metadata.

Each row uses ✅/⚠️ status and short notes on how this provider behaves (or that capability comes from the OpenFeature SDK). DependencyInjection (experimental) is the only ⚠️ row, with links to #48 and #49; a legend clarifies the status symbols.

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