Add comments clarifying what files do

[owencm]

There is a lot of stuff going on in this repo. I think it would be really helpful to have at least a few sentences at the start of files such as Dockerfile, app.yaml and gulpfile.js (and probably many other files) to clarify what they do.

I think that would really help people trying to orient their way around this and work out what to modify / rip out for their own variation.

[addyosmani]

Agree completely. I think an appendix walking through the responsibilities of our different files (or comments in files) would be valuable.

[addyosmani]

Would there be value in us having an appendix.md file that walks through what each directory / roughly what the important files in the repo correspond to? (cc @gauntface for his thoughts too)