Quick guide published

A new file is added to documentation: this guide is the manual our users receive inside DGT: contrarily to the web site, it also details things which already exist in standard OmegaT, with no focus on differences.

The version published here contains some differences with the internal one, for example, links to the downloadable files. However it is still possible that a few parts of this manual are still specific to internal version, sorry for inconvenience.