Update README with the fastest way to lint multiple pug files

[koresar]

I took me a while to find how to lint many-many files very-very fast. That is my finding. I hope someone would find this helpful.

[ci-reporter]

The build is failing

✨ Good work on this PR so far! ✨ Unfortunately, the Travis CI build is failing as of 02add6b. Here's the output:

npm test

> pug-lint@2.6.0 pretest /home/travis/build/pugjs/pug-lint
> npm run docs && npm run schema && npm run lint


> pug-lint@2.6.0 docs /home/travis/build/pugjs/pug-lint
> pliers buildRuleDocs

/home/travis/build/pugjs/pug-lint/node_modules/fs-extra/lib/index.js:5
  ...require('./fs'),
  ^^^

SyntaxError: Unexpected token ...
    at createScript (vm.js:56:10)
    at Object.runInThisContext (vm.js:97:10)
    at Module._compile (module.js:549:28)
    at Object.Module._extensions..js (module.js:586:10)
    at Module.load (module.js:494:32)
    at tryModuleLoad (module.js:453:12)
    at Function.Module._load (module.js:445:3)
    at Module.require (module.js:504:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (/home/travis/build/pugjs/pug-lint/node_modules/docco/docco.js:204:8)

npm ERR! Linux 4.15.0-1028-gcp
npm ERR! argv "/home/travis/.nvm/versions/node/v6.17.1/bin/node" "/home/travis/build/pugjs/pug-lint/node_modules/.bin/npm" "run" "docs"
npm ERR! node v6.17.1
npm ERR! npm  v3.10.10
npm ERR! code ELIFECYCLE
npm ERR! pug-lint@2.6.0 docs: `pliers buildRuleDocs`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the pug-lint@2.6.0 docs script 'pliers buildRuleDocs'.
npm ERR! Make sure you have the latest version of node.js and npm installed.
npm ERR! If you do, this is most likely a problem with the pug-lint package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     pliers buildRuleDocs
npm ERR! You can get information on how to open an issue for this project with:
npm ERR!     npm bugs pug-lint
npm ERR! Or if that isn't available, you can get their info via:
npm ERR!     npm owner ls pug-lint
npm ERR! There is likely additional logging output above.

npm ERR! Please include the following file with any support request:
npm ERR!     /home/travis/build/pugjs/pug-lint/npm-debug.log

I'm sure you can fix it! If you need help, don't hesitate to ask a maintainer of the project!

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