Added new section to the git-svn getting started section that provides a
custom git script called git-svnup which handles all of the work of
using the git-mirrors/keeping the git-svn numbers in sync.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173472 91177308-0d34-0410-b5e6-96231b3b80d8