icon cache scriplet guideline update

[Bill Nottingham]

Matthias Clasen (mclasen at redhat.com) said: 
> > if we have to run it it in the rpm transaction.  it should be run only once 
> > per rpm transaction not once per rpm. 
> 
> That's one thing to consider for the "revive rpm development" effort.

The logical place to do things like this would be in a yum plugin.
However, that's going to run into the 'don't want to use yum' problem,
so you start having to tie hooks into rpm itself, which leads you to
macro-ization of everything, %define __post_transaction_hook, a mechanism
to pass arbitrary transaction data from a commandline program to another
commandline helper... fun.

Bill