modification: doc/README_*.txt much simpler with link to piwigo.org for
installation and upgrade guides.
modification: latest_version has changed its path on the website to download
directory.
deletion: doc/ChangeLog was useless and outdated.
git-svn-id: http://piwigo.org/svn/branches/2.0@3132 68402e56-0260-453c-a942-63ccdbb3a9ee