Fixed two issues in the 'Start a Go instance in your app' section

- See Issue 24 @ https://github.com/docker-library/golang
- Altered the explanation to indicate the ONBUILD trigger does a go install rather than go build via the go-wrapper script.
- Added some content on the CMD instruction.
- Changed order so it is build and run image so it matches the commands that follow.
I have restored the CMD ["app"] instruction to the Dockerfile. I don't care for the pretending but it does allow someone to get up and running immediately and hints at how you can pass parameters to the app.

I do think we need an explanation of the go-wrapper because it is unexpected when i read the golang:1.3.1-onbuild image dockerfile and the simple explanation in the "Start a Go instance in your app". This would then be an opportunity to explain the .godir option.