README: move install and dev infos into main README
README: add FM7 in plugins list
documentation: delete obsolete readme files

 - all those infos are in main README.md now
Merge pull request #4 from gusano/topic/improve_documentation

Merge documentation updates from Yvan Volochine: move install, dev and package infos into README; delete obsolete VERSION file (and INSTALL, readme_devs.txt and readme_package.txt)