New repository for Strawberry Perl Portable Edition Packages for
FusionInventory Agent.

Last December 11 was published in the GitHub blog the news Goodbye,
Uploads' (https://github.com/blog/1302-goodbye-uploads). That is the
reason why it is needed to change the Strawberry Perl Portable Edition
Packages for FusionInventory Agent repository. The next March 10, files
located in https://github.com/tabad/fusioninventory-agent-windows-
installer/downloads will not be available.

I have choose SourceForge for this purpose.
Update and improve documentation

These are the changes:

 - New Strawberry Perl Portable Edition repository. (URL updated)

 - New experimental repository for FusionInventory Agent Windows
   Installers.

 - The term 'Strawberry Perl Package' has been changed by 'Strawberry
   Perl Portable Edition Package'.

 - More information regarding with when it's necessary to build a
   new Strawberry Perl Portable Edition Package.

 - New section 'Repositories'.

 - Removed some spelling errors.