Quick start and Usage docs

[Sébastien Pierre]

Hi all,

I carefully read all your suggestions, and generally agreed the
modifications. Thank you Brian and Christian for your careful review,
and the clarity of your descriptions.

As a summary :

- There is now a white background by default (solves transparency
problems)
- The grey on grey shades are more contrasted
- 90% of Brian and Christian's modifications were applied (see below)
 
However, for quick start:

- [Quick Start] I did not really understand Brian's suggestion: on the
"Public Repo" / "Working Repo" diagram: <<You might want to move the
annotation "working dir" to the "Working Repo" portion.  Quite often a
public repo will not have its working dir populated at all.>>.

and for usage :

- I could not put the "working dir" clarification of Christian because
of space and complexity problems

I will post the updated versions of the documents soon !

 -- Sébastien

PS: I had some feedback that the orange was too flashy. Had some of you
the same impression ?