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

[flyingzumwalt]

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
• go help gopath
• Understanding Golang Packages
• Video: Writing, building, installing, and testing Go code

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