Add a README

[KyleTryon]

Add a readme

[forceedge01]

+1. If you're going to put the effort in making something for others, makes sense to include a sensible readme in here. The current documentation on the published on circle ci doesn't go on to explain what continue-config is supposed to be. Going all over the place to see how this works, not great.

[mathaeus]

Heavy +1 on this one.

@KyleTryon I only figured out how this orb really works by the help of examples of other people like this one: https://gist.github.com/ryanpedersen42/c867acd9b0837789a92836e8f5bd8b55

[njsaunders]

+1 - same

[elliotforbes]

Closing as the readme has been added.