Documentation: Remove mentions of git-svnimport.
git-svnimport is no longer supported, so don't mention it in the
documentation. This also updates the description, removing the
historical discussion, since it mostly dealt with how it differed from
svnimport. The new description gives some starting points into the
rest of the documentation.
Noticed by Jurko Gospodnetić <jurko.gospodnetic@docte.hr>
Signed-off-by: Brian Gernhardt <benji@silverinsanity.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>