add docker compose for sample apps

[fmgornick]

Hey I don't know if you're doing any more development on this project, but I figured it'd be nice to have a feature to allow people to run/stop the example docker containers in one line. I'm sure you already know how docker-compose works so if you just decided it'd be better to leave out an extra yaml file then please feel free to reject, but otherwise I just added the one file and a couple lines in the readme if you think it'd be a nice addition.

Anyway, I'd also like to say thanks, because this project really helped me understand how envoyproxy's go-control-plane works, I've been working on a project of my own and was feeling pretty intimidated from envoys lack of examples.