git-svn: Document branches with at-sign(@).
git-svn: Document branches with at-sign(@).

git svn sometimes creates branches with an at-sign in the name
(branchname@revision). These branches confuse many users and it is a FAQ
why they are created. Document when git svn creates them.

Signed-off-by: Sebastian Leske <10124697c554a0e913a6f2f18a03410e0eacc57a@sleske.name>
Acked-by: Eric Wong <8c2951295f8292b0c03727bc4652981172298eae@yhbt.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git