doc: Convert adding a fop howto to markdown

Change-Id: Ia0a9ad228dcf69f40befc58fff7a56fffcfe915b
Signed-off-by: Lubomir Rintel <lubo.rintel@gooddata.com>
Reviewed-on: http://review.gluster.org/5487
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>