Example could be more idiot-proof - go paths, protoc installation

[jtippett]

As a total go n00b it took me an embarrassing amount of time to get the toy example app working (regenerating the protos). I suggest two improvements:

1. Better instructions for installing the golang protobuf compiler. The example app points us at https://github.com/golang/protobuf, which may as well be in Klingon to a non-gopher. For mac, all you need to do it seems is brew install protobuf and protoc is magically present (I had it already - the "golang compiler" step was a red herring). Installing golang itself is also, obviously, needed.

2. Golang appears to have its own special path for executables, and go get installs things in ~/go/bin, and if you don't have that in your PATH then the build will fail. For me, it was as simple as adding export PATH=/Users/james/go/bin:$PATH to the bottom of my .zshrc and re-sourcing it.

For those not accustomed to golang ways, these may be rather confusing (they were for me). Thanks!