Update readme.md

Rewritten and formatted as Markdown document. Removed legacy French translation.
Throw an exception if there is not enough memory. Fixes #299
Merge branch '3.6' of github.com:impresspages/ImpressPages-CMS into 3.6
Access themes from multiple folders #300
Autoinstall theme plugins
rename dirs to folders
allow dash in theme name
Merge branch '3.6' of github.com:impresspages/ImpressPages-CMS into develop