Docs(contributing): Specify global installation of jest while running single tests #775

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

What is the current behavior?
Global installation instance of jest cli is missing

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

What is the expected behavior?
It should mention to install the jest cli https://jestjs.io/docs/en/getting-started.html#running-from-command-line

If this is a feature request, what is motivation or use case for changing the behavior?
People should know that they need jest-cli

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

Sending a quick PR. 😄