Document comparisons option for minifying Tangram

[hanbyul-here]

• I faced the problem that Tangram requires specific options to be minified with uglify-js when I was bundling Tangram for mapzen.js.
• Now this info is in the comment of react-webpack-tangram-boilerplate repo I think it is better to keep this info more formally documented since uglify is very common, also not React-Webpack specific command.