docs: document Windows/PowerShell npx invocation

[mvanhorn]

Summary

Documents the working npx invocation for Windows/PowerShell users in the README installation section.

Why this matters (#93)

Running npx @google/design.md lint DESIGN.md in PowerShell on Windows produces no output, and in some cases Windows opens the DESIGN.md file in a Markdown editor instead of running the CLI. The root cause is the design.md bin name: the .md suffix collides with the Windows Markdown file association during command resolution, so the token resolves to a file rather than the npm shim.

The package already ships a dot-free designmd bin alias pointing at the same entrypoint, and the README already documents it for the package.json-script case. The gap was the direct-npx case that #93 is specifically about.

Changes

• README: added a Windows/PowerShell note right after the direct-npx install example, pointing users at npx -p @google/design.md designmd lint DESIGN.md and briefly explaining the .md file-association collision. Wording is kept consistent with the existing designmd-alias guidance for the scripts case.

Docs-only; no code touched.

Testing

• Verified against packages/cli/package.json: the bin map exposes both design.md and designmd, both pointing at ./dist/index.js, so the documented designmd alias is accurate and resolves to the same entrypoint.
• Confirmed the macOS/Linux npx @google/design.md lint DESIGN.md instructions are unchanged.
• Markdown code fences are balanced and the new note is consistent with the pre-existing designmd-alias tip.

Fixes #93.

[davideast]

LGTM! Thank you @mvanhorn! I've been looking into solutions here and I think this is absolutely the best start for now.