user-manual: detached HEAD
user-manual: detached HEAD

Add a brief mention of detached HEADs and .git/HEAD.

Signed-off-by: "J. Bruce Fields" <51738506c1b2ccb0761f23bdc612c93babf738ea@citi.umich.edu>
Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

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