Add docs for customizing the generator templates

__tldr;__ Document how to customize templates within the `.donejs/templates/` folder.

> This was discussed on a [recent live stream (38:36)](https://youtu.be/coSEECcyi00?t=38m36s), a [previous live stream (56:06)](https://youtu.be/ieZJrPwSZYQ?t=56m06s), and at a [contributors meeting (55:34)](https://youtu.be/95OX6-qii24?t=55m34s).

# The Problem

Often times people will want to use the generators, but have to then change the generated files because their project is customized in some way. One example might be using mocha instead of QUnit. The donejs-mocha package will create templates in `.donejs/templates/` that will be used instead of the standard templates.

A user can do the same thing, by copying the standard templates into the `.donejs/templates/` folder and modifying them.

This is a cumbersome process because:

* You have to manually copy them from the generator-donejs project. Gross!
* There is no documentation on what you are supposed to do (including the above).

# The Solution

This is a two part solution.

## donejs customize

Running `donejs customize` (or some other command, I don't care) will create the `.donejs/templates/` folder for you, and populate it with all of the default templates.

## Document customizing these on donejs.com

Under __Guides__ add a __Customizing Generator Templates__ guide, that walks through step by step of running `donejs customize` and then customizing one of the templates.