docs: install via bundle + one-click nzip: link #13

The install section now leads with the two steps people should use: install bundle if you don't yet have it (github.com/pipobscure/bundles), then click the nzip: link for davsync. bundle 0.1.6 registers the nzip: scheme handler when it installs itself, so the link opens bundle and installs the tool in a window (bundle install --window --). The terminal form stays as the alternative, switched to the listing name where it still pointed at a URL.