Explain syntax highlighting in Installfest docs

At the Oct. 4 Installfest in San Francisco, a student asked, "What are all these weird colors?" when she was reading the Gemfile example. I noticed we explain syntax highlighting in the Ruby curriculum; how about we do the same for the student's possibly first encounter with a code editor?

I copied the explanation of syntax highlighting from the Ruby curriculum, and mentioned that's what's creating the colors in the code sample. This might be a good place for a partial instead.