update README

Author: archmoj

README.md

@@ -75,8 +75,29 @@ Starting in `v1.15.0`, plotly.js ships with several _partial_ bundles (more info
 
 Starting in `v1.39.0`, plotly.js publishes _distributed_ npm packages with no dependencies. For example, run `npm install plotly.js-geo-dist` and add `import Plotly from 'plotly.js-geo-dist';` to your code to start using the plotly.js geo package.
 
-If none of the distributed npm packages meet your needs, you create custom bundle of desired trace modules e.g. `pie` and `sunburst` using
-`npm run partial-bundle pie sunburst name=custom`
+If none of the distributed npm packages meet your needs, and you would like to manually pick which plotly.js modules to include, you'll first need to run `npm install plotly.js` and then create a *custom* bundle by using `plotly.js/lib/core`, and loading only the trace types that you need (e.g. `pie` or `choropleth`). The recommended way to do this is by creating a *bundling file*. For example, in CommonJS:
+
+```javascript
+// in custom-plotly.js
+var Plotly = require('plotly.js/lib/core');
+
+// Load in the trace types for pie, and choropleth
+Plotly.register([
+    require('plotly.js/lib/pie'),
+    require('plotly.js/lib/choropleth')
+]);
+
+module.exports = Plotly;
+```
+
+Then elsewhere in your code:
+
+```javascript
+var Plotly = require('./path/to/custom-plotly');
+```
+
+Alternatively you could browserify a custom bundle of desired trace modules e.g. `pie` and `choropleth` using
+`npm run partial-bundle pie choropleth name=custom`
 
 
 ## Building plotly.js