Update README

 * Add screenshots
 * Use Mediacrush as a CDN for images to reduce load on sebsauvage.net
 * Improve page layout/sections
 * Reorder list of features
 * Add links to wiki and bug tracker
 * Add proper download links for stable/dev channels
 * Improve copyright/license notice

Preview at https://github.com/nodiscc/Shaarli/tree/new-readme
Comparison here https://cdn.mediacru.sh/O-H8masv_Wnj.jpg