#258 Update External Libraries documentation using 7zip

[Miro Halas]

The External libraries documentation refers to using
Windows built-in extraction tool to unpack the
libraries. This is very clunky since the libraries
needs to be unpack one by one. Update the documentation
to suggest using 7zip (http://www.7-zip.org/) which is
free tool and allows to unpack all the external
libraries at once by selecting them all, right clicking
on them and selecting Extract here.