docs: Add downloads badge from npm

[anshumanv]

Do you want to request a feature or report a bug?
Docs/feat

What is the current behavior?
No downloads badge present

If the current behavior is a bug, please provide the steps to reproduce.
NA

What is the expected behavior?
A cool badge denoting the downloads can be present.

If this is a feature request, what is motivation or use case for changing the behavior?
Users can see the popularity without visiting npm

Please paste the results of webpack-cli info here, and mention other relevant information such as programming language.

webpack-cli info

  System:
    OS: macOS 10.14.3
    CPU: (4) x64 Intel(R) Core(TM) i5-5250U CPU @ 1.60GHz
  Binaries:
    Node: 10.14.1 - /usr/local/bin/node
    Yarn: 1.12.3 - /usr/local/bin/yarn
    npm: 6.4.1 - /usr/local/bin/npm
  Browsers:
    Chrome: 72.0.3626.119
    Safari: 12.0.3
  npmPackages:
    webpack: 4.x.x => 4.29.6
    webpack-dev-server: ^3.1.10 => 3.2.1