Add uv tool install instructions to README

[spboyer]

Summary

The README currently only shows installation using uvx --from git+https://github.com/github/spec-kit.git specify, but uv tool install provides a more permanent installation method that's becoming the preferred approach for installing Python tools.

Current Installation Method

uvx --from git+https://github.com/github/spec-kit.git specify init <PROJECT_NAME>

Proposed Addition

The README should also include instructions for installing with uv tool install, which provides several benefits:

# Install once, use everywhere
uv tool install specify-cli --from git+https://github.com/github/spec-kit.git

# Then use normally
specify init <PROJECT_NAME>
specify check

Benefits of uv tool install

1. Persistent Installation: The tool stays installed and available in PATH until explicitly uninstalled
2. No Need for Aliases: Users don't need to create shell aliases for convenience
3. Better Tool Management: Can list, upgrade, and uninstall tools easily with uv tool list, uv tool upgrade, uv tool uninstall
4. Cleaner Shell Configuration: No need to modify .zshrc or .bashrc files

Suggested Changes

Add a new section to the README under "Get started" or create an "Installation" section that shows both methods:

Installation Options

Option 1: Persistent Installation (Recommended)

uv tool install specify-cli --from git+https://github.com/github/spec-kit.git

Then use the tool directly:

specify init my-project
specify check

Option 2: One-time Usage

uvx --from git+https://github.com/github/spec-kit.git specify init <PROJECT_NAME>

This would help users choose the installation method that best fits their workflow.