Note CSS in npm installation instructions (#4021)

* Note CSS in npm installation instructions

I saw that there was no mention of the required CSS file outside of the "Using Mapbox GL JS with a `<script>` tag" section. I thought it might be a good idea to include it in each installation section, for readers who are skipping to the section that concerns them.

* Add node_modules to CSS paths

* Update README.md

README.md

@@ -62,6 +62,8 @@ var map = new mapboxgl.Map({
 });
 ```
 
+Add the CSS file at `node_modules/mapbox-gl/dist/mapbox-gl.css` or `https://api.tiles.mapbox.com/mapbox-gl-js/v0.31.0/mapbox-gl.css`.
+
 ## Using Mapbox GL JS with other module systems
 
 Since our build system depends on Browserify, to use Mapbox GL with any other module bundlers like [Webpack](https://webpack.github.io/), [SystemJS](https://github.com/systemjs/systemjs), you have to require the distribution build instead of the package entry point:
@@ -76,6 +78,8 @@ If you're using the ES6 module system (e.g. with [Rollup](https://github.com/rol
 import mapboxgl from 'mapbox-gl/dist/mapbox-gl.js';
 ```
 
+Add the CSS file at `node_modules/mapbox-gl/dist/mapbox-gl.css` or `https://api.tiles.mapbox.com/mapbox-gl-js/v0.31.0/mapbox-gl.css`.
+
 ## Third Party Projects
 
 These projects are written and maintained by the GL JS community. Feel free to open a PR add your own projects to this list. We :heart: third party projects!