quick effort at explaining/introducing companies features in Readme.md -...

... addresses https://github.com/intercom/intercom-rails/issues/61

@erskingardner - this is a quick effort at addressing #61 

(view properly https://github.com/intercom/intercom-rails/blob/ae01716f6069230c0f1b7cbebc01cdf717dfc4b2/README.mdown)

We need to do a more complete overhaul of the readme here (or shrinking it and linking to better docs elsewhere) - but how does this address things for now?