[Lesson: Install Go] Add links to info about GOPATH and setting environment variables

Under **"Check your Installation"**, the Getting Started page says

> Create a directory to contain your workspace, $HOME/work for example, and set the GOPATH environment variable to point to that location.

This is really confusing for someone who doesn't already understand environment variables.

Here are some references to explain these ideas:
- [Golang Docs: The GOPATH environment variable](https://golang.org/doc/code.html#GOPATH)
- [go help gopath](https://golang.org/cmd/go/#hdr-GOPATH_environment_variable)
- [Understanding Golang Packages](http://thenewstack.io/understanding-golang-packages/)
- Video: [Writing, building, installing, and testing Go code](https://www.youtube.com/watch?v=XCsL89YtqCs)

Setting environment Variables on Mac OSX:
http://stackoverflow.com/questions/7501678/set-environment-variables-on-mac-os-x-lion