add documentations

Signed-off-by: ChengZi <chen.zhang@zilliz.com>

packages/core/CONTRIBUTING.md

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+# Contributing to @code-indexer/core
+
+Thanks for your interest in contributing to the CodeIndexer core package!
+
+> 📖 **First time contributing?** Please read the [main contributing guide](../../CONTRIBUTING.md) first for general setup and workflow.
+
+## Core Package Development
+
+This guide covers development specific to the core indexing engine.
+
+## Development Workflow
+
+### Quick Commands
+```bash
+# Build core package
+pnpm build:core
+
+# Watch mode for development
+pnpm dev:core
+
+# Type checking
+pnpm typecheck
+```
+
+### Making Changes
+
+1. Create a new branch for your feature/fix
+2. Make your changes in the `src/` directory
+3. Follow the commit guidelines in the [main guide](../../CONTRIBUTING.md)
+
+## Project Structure
+
+- `src/indexer.ts` - Main CodeIndexer class
+- `src/embedding/` - Embedding providers (OpenAI, VoyageAI)
+- `src/vectordb/` - Vector database implementations (Milvus)
+- `src/splitter/` - Code splitting logic
+- `src/types.ts` - TypeScript type definitions
+
+## Guidelines
+
+- Use TypeScript strict mode
+- Follow existing code style
+- Handle errors gracefully
+
+## Questions?
+
+- **General questions**: See [main contributing guide](../../CONTRIBUTING.md)
+- **Core-specific issues**: Open an issue with the `core` label