improve the README file

[garaud]

I don't understand what the project does. OK, it's a "compagnon", but what does it mean?

Just a few "pythonic" bindings to the C++ CGAL library?

Please write a short description.

[vmora]

I believe it's done, comments ?

[garaud]

Just a tiny typo. See c713476#r27859250

Looks good for me! Thanks.