Working on Doc

[Rému Devreux <remudevreux gmail com>]

Hello, I worked a bit on updating Gnote documentation. 

* mallard format
* minor reorganization
* brief sentence about gnome3 messaging tray vs fallback mode

If there are missing points i did not notice, or if you disagree with
organization, please provide feedback.
So it's there : http://py.luyten.fr/Publique/gnote/html0.3/

It's quite uncompleted. No add-in yet , very very light "common
problems" section, no translation...

Regards,
Pierre-Yves