git-svn: Recommend use of structure options.

Document that when using git svn, one should usually either use the
directory structure options to import branches as branches, or only
import one subdirectory. The default behaviour of cloning all branches
and tags as subdirectories in the working copy is usually not what the
user wants.

Signed-off-by: Sebastian Leske <sebastian.leske@sleske.name>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>