doc: Convert the write-behind translator documentation to markdown

Change-Id: I213e230d283cd43bd1c2b2948882ab36f9170608
Signed-off-by: Lubomir Rintel <lubo.rintel@gooddata.com>
Reviewed-on: http://review.gluster.org/5482
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>