doc: How to generate and read statedump

Thanks to Poornima G's help with iobuf
Section explanation.

Change-Id: I17737fdbd1f402914f7e67fb4047f5c26ea5c36c
BUG: 1118309
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/8288
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Reviewed-by: Prashanth Pai <ppai@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>