[Encore] Add docs on how to add extra webpack config

[weaverryan]

Encore doesn't support adding all of Webpack's options... because many are easy enough for you to add yourself. For example (from symfony/webpack-encore#35), if you want to add watchOptions, I think we should encourage you to add those yourself:

// webpack.config.js

var Encore = require('@symfony/webpack-encore');

// all Encore config here

var config = Encore.getWebpackConfig();

config.watchOptions = { poll: true, ignored: /node_modules/ };

module.exports = config;

[weaverryan]

Other examples:

• webpack resolve alias webpack-encore#37
• Add loader for Typescript webpack-encore#9 (comment)