adding software documentation

[Amy Vastola]

Reviewing this section, and it's pretty accurate (I can only review this 
as the new user that I am) , but sometimes the pieces don't really paint 
a full picture. For example,  explanation of the repositories, I assume 
to make people aware of what they are taking on when they select a 
software package. But I think a mention is needed of where to look to 
find out which repository any particular application comes from. So you 
get this  great explanation of the software universe, but  need to know 
how to  apply  your understanding when you are in the package manager 
dialogue and about to select software. I see the way one would filter 
for different software categories in the two desktop package managers, 
and will add that, but can't find any mention of software selection in 
the APT-GET manual.

Amy Vastola