Improve tutorial

[fryguybob]

(Imported from http://code.google.com/p/diagrams/issues/detail?id=74. Original issue from byor...@gmail.com on March 12, 2012, 01:59:57 AM UTC)

There are a number of ways that the current tutorial could be improved:

1. Include more features. E.g. it currently doesn't even talk about trails or paths at all.
2. It would be nice to organize some of it (e.g. perhaps some new material, or perhaps including a rewrite of some of the current material) around a coherent example, e.g. "Suppose we'd like to make this nice diagram here. The next three sections will walk you through the steps and concepts we need to create it.".

This could make a good project for someone wanting to contribute and confident in their writing skills but not as confident in their Haskell skills -- it would be a concrete way to contribute that wouldn't require a lot of Haskell experience, but also a way to gain some experience and understanding at the same time.