Add claude.md with info on the repo structure, to help Claude know where things are (#840)

* Add `claude.md` with info on the repo structure, to help Claude know where things are

* Feedback from @cscheid

* One more note about `writer`

* Add sections on development workflow and testing

Author: juliasilge

claude.md

@@ -0,0 +1,64 @@
+# Quarto Development Guide
+
+## Project Overview
+
+Quarto is an open-source scientific and technical publishing system built on [Pandoc](https://pandoc.org). This repository contains the source code for various parts of the Quarto ecosystem, with the main CLI implementation housed in a separate repository ([quarto-cli](https://github.com/quarto-dev/quarto-cli)).
+
+### Main Components
+
+- **VS Code Extension**: The primary VS Code extension for working with Quarto documents
+- **Writer**: An experimental web-based editor for Quarto documents (not used in production yet)
+- **LSP**: Language server for Quarto documents
+- **Core Packages**: Shared libraries used across multiple components
+
+## Development Workflow
+
+Each component has specific development guidelines. Refer to the corresponding CONTRIBUTING.md files:
+
+- VS Code extension: [apps/vscode/CONTRIBUTING.md](apps/vscode/CONTRIBUTING.md) - Contains detailed instructions for building, debugging, and releasing the extension
+
+## Repository Structure
+
+The repository is organized as a monorepo using Yarn workspaces and Turborepo for build orchestration:
+
+- `apps/`: Contains standalone applications
+  - `vscode/`: VS Code extension for Quarto
+  - `writer/`: Experimental web-based Quarto editor (not in production, ignore this for now)
+  - `lsp/`: Language Server Protocol implementation
+  - `panmirror/`: WYSIWYG editor component
+- `packages/`: Contains shared libraries
+  - `core/`: Core functionality shared across packages
+  - `editor-*/`: Editor-related packages
+  - `quarto-core/`: Quarto-specific core functionality
+  - Other utility packages
+
+## Build System
+
+Quarto uses [turborepo](https://turbo.build/) to manage the monorepo build process:
+
+- `turbo.json`: Defines the pipeline configuration for common tasks
+- Common commands:
+  - `yarn build`: Builds all packages and applications
+  - `yarn dev-writer`: Runs the writer app in development mode
+  - `yarn dev-vscode`: Runs the VS Code extension in development mode
+  - `yarn lint`: Runs linters across all workspaces
+  - `yarn build-vscode`: Builds only the VS Code extension and its dependencies
+
+The turborepo pipeline helps optimize build times by caching build artifacts and respecting the dependency graph between packages.
+
+## Testing
+
+Testing procedures vary by component:
+
+- VS Code extension: Run `yarn test-vscode` to compile test files and run them with the vscode-test CLI
+- Other components have specific test commands defined in their respective package.json files
+
+
+## Additional Resources
+
+- [Quarto Website](https://quarto.org)
+- [Extension on Microsoft marketplace](https://marketplace.visualstudio.com/items?itemName=quarto.quarto)
+- [Extension on Open VSX Registry](https://open-vsx.org/extension/quarto/quarto)
+- [Quarto GitHub Organization](https://github.com/quarto-dev)
+
+# Instructions