Roadmap/en

Here are a few things we want to improve in the future:
 * Content incremental update
 * Multi ZIM file search engine
 * Software auto update for the desktop version
 * *.kiwix (index+content) format integration
 * ePub ebook format support
 * Httrack integration with zimwriter
 * Speech synthesizer to read articles
 * Better index path manipulation
 * Fully P2P distribution solution
 * OPDS Support