Updated Readme.md for npm package command

[nikhilknoldus]

Added npm install command for installing the package, if user do not does so, it shoes an error while running webpack command, says: jshit module can not be found.

Please check if the PR fulfills these requirements

• [ ] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

• [x] Bugfix
• [x] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here) Shows error when application runs, says jshit module can not be found.

What is the new behavior?

It does not show such error.

Does this PR introduce a breaking change?

• [x] Yes
• [ ] No

If this PR contains a breaking change, please describe the following...

• Impact:
• Migration path for existing applications:
• Github Issue(s) this is regarding:

Other information:

[jsf-clabot]

All committers have signed the CLA.

[nikhilknoldus]

Hi @sokra can you please have a look. Thanks !!

[kostasmanionis]

Closing in favor of https://github.com/webpack/jshint-loader/pull/31