Proposed additions to Maxima's reference manual

Hello Mario,

> In summary, I've added the new section 'Additional packages' with the
> following contents:
>
> Additional packages
>
> * zeilberger::                  Functions for hypergeometric summation.

Looks good to me -- a couple of suggestions --
(1) organize the list of additional packages alphabetically.
(A thematic organization would be better, but after scanning
the list I don't see any obvious themes.)
(2) merge the list of contrib packages into "Additional packages"
(so there is only 1 list). The distinction between contrib and non-
contrib packages is largely historical and not very useful to the
reader.

Thanks for your work on this topic,

Robert