Give more background on the development environment

[xMartin]

A question from the feedback form:

the functions we used to draw are built in ? Is there a built in library ? Do every javascript enviroinment offer the same set of basic functions ?

We should explain a little bit.

[xMartin]

the functions we offer in the excercises are not directly canvas but a thin custom layer on top. that's the issue

[arky]

@xMartin Thanks for clearing that up. I think I'll find a simple way of explain that in the first page and then add reminders in the exercises.