code: function checkNpmVersion() {
    return execa.shell('npm -v')
      .then(({ stdout }) => stdout)
      .then((v) => {
        if (v && !(v.indexOf('3') === 0)) {
          log('Use npm v3.10.10 for unobtrusive symlinks: npm i -g npm@3.10.10');
        }
      });
  }
docs: check npm version and show warning if needed