Add channels example

[fritzblue]

Thank you for throwing this repo together! It is an awesome example of combining two compelling technologies to create something greater than the sum of its parts.

It would be awesome to add a basic example of how Phoenix channels would fit into the mix (perhaps a simple GraphQL-based chat example).

[joshprice]

Glad you found it useful. :) Subscriptions are still somewhat experimental and not fully spec'd out last time I checked. However this is definitely something we'll be looking at soon, since Channels should hopefully make implementing subscriptions quite straightforward.

In the meantime feel free to investigate this if it's of interest to you.