Document the HTTP transport protocols

Signed-off-by: Shawn O. Pearce <[email protected]>
Revised-by: Tay Ray Chuan <[email protected]>
Signed-off-by: Nguyễn Thái Ngọc Duy <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: spearce