Update docs to emphasize selector can be a string or a DOM node

When instantiating a chart, the docs suggest that you have to pass in a d3 css string selector as the input parameter called `parent`. When creating the chart, `parent` is passed in as the input parameter to d3's select function. Because [d3's select works on DOM nodes](https://github.com/mbostock/d3/wiki/Selections#d3_select) in addition to string selectors,  `parent` could be a DOM node too.

Explicitly pointing out the ability to instantiate a chart with a DOM node is useful for anyone trying create a chart before injecting it into the DOM tree (for example in a Backbone.js view).