etch upgrade release notes, chapter 4.5.3: mention localepurge, too?

[Andreas Mohr <andi@lisas.de>]

Hi all,

I'd think it'd be useful to add a note about localepurge at the
"4.5.3 Make sure you have sufficient space for the upgrade" chapter
(purging unused locales freed a whopping 240MB on this machine here
on Debian stable).

Possibly with a note saying that one should only do this if very
space-starved (perhaps purging locales might cause upgrading issues?),
and thus maybe list this suggestion last.

Thanks,

Andreas Mohr