docs: add link to NPM in monthly downloads badge (#318)
docs: add link to NPM in monthly downloads badge (#318)

- previously it just linked to the image itself; better to link to the
  source of the stats which is NPM
  - duplicates the other badge's link, but nbd, better than linking to
    an image imo
ezolenko/rollup-plugin-typescript2,ezolenko/rollup-plugin-typescript2