docs: update feature list with more detailed descriptions

The README.md feature list has been enhanced with more precise and detailed
descriptions of the plugin's capabilities. Key updates include:

- Clarified native Copilot Chat integration and supported models
- Added details about smart embeddings system
- Improved explanation of context sharing mechanics
- Enhanced description of the prompt system
- Refined explanations of UI and provider features

Author: deathbeam

README.md

@@ -20,13 +20,14 @@ https://github.com/user-attachments/assets/8cad5643-63b2-4641-a5c4-68bc313f20e6
 
 CopilotChat.nvim is a Neovim plugin that brings GitHub Copilot Chat capabilities directly into your editor. It provides:
 
-- 🤖 Full GitHub Copilot Chat integration with selective context sharing - only sends what you want to share
-- 💻 Smart workspace context awareness (buffers, files, git diffs, URLs)
-- 🔌 Extensible provider system (supports custom LLM providers like Ollama)
-- 📝 Rich chat interface with code completions and diff acceptance
-- 🎯 Built-in prompts for common tasks (explain, review, fix, optimize, docs, tests)
-- 🔄 Context-aware responses that understand your code and workspace
-- ⚡ Efficient token usage with optional tiktoken support
+- 🤖 Native GitHub Copilot Chat integration with official model and agent support (GPT-4o, Claude 3.5 Sonnet, Gemini 2.0 Flash, and more)
+- 💻 Rich workspace context powered by smart embeddings system
+- 🔒 Explicit context sharing - only sends what you specifically request, either as context or selection
+- 🔌 Modular provider architecture supporting both official and custom LLM backends (Ollama, LM Studio, and more)
+- 📝 Interactive chat UI with completion, diffs and quickfix integration
+- 🎯 Powerful prompt system with composable templates and sticky prompts
+- 🔄 Extensible context providers for granular workspace understanding (buffers, files, git diffs, URLs, and more)
+- ⚡ Efficient token usage with tiktoken optimization
 
 # Requirements