[LICENSE] Add the CeCILL V2.1 license text.

The applicable license is in French.
You can read an English translation here: http://www.cecill.info/licences/Licence_CeCILL_V2.1-en.html
The French and English version have the same legal value (see 11.5)

Signed-off-by: Nicolas SAPA <nico@ByMe.at>