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.
Removed the CMD instruction from the Dockerfile and restored the CMD explanation paragraph which is a simplification and not correct as yosifkit pointed out previously, but this simpler explanation helps getting up and running as fast as possible ? 

Should we include a go-wrapper and .godir section in a new pull request ? Could cause more confusion ?
 
Re-ran the root update.sh file and it has updated a number of README.md files i.e. mongo/README.md, I'm guessing this was due to tianon's last commit efbf65d
I'm guessing I should wait until this is fixed on master so I can merge before pushing to my fork ?