cmd/tsconnect: add README to generated NPM package

Makes the landing page at https://www.npmjs.com/package/@tailscale/connect
look slightly nicer.

Fixes #5976

Signed-off-by: Mihai Parparita <mihai@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f