LP#1527731: Document enabling stable hash key ordering in mod_perl

LP#1527731: Document enabling stable hash key ordering in mod_perl

Since several Evergreen features within mod_perl depend on hash key ordering,
and OpenSRF currently installs mod_perl, we'll document the change needed here
instead of in Evergreen.  When the mod_perl configuration moves to the
Evergreen repo, we can move the documentation.

Signed-off-by: Mike Rylander <mrylander@gmail.com>