[FAB-8228] Documenting the script for Adding an Org

Script developed for Adding an Org. This documents that.
Also name of doc changed to reflect what it is and to make room
for coming documentation on channel configs.
Also small tidying up/grammar edits.
Staged here: http://joestaging-fabric.readthedocs.io/en/latest/channel_update_tutorial.html
[ci-skip]

Change-Id: I8148ddf9a8ae59c085cf829c63e78f1b1666c1c3
Signed-off-by: joe-alewine <Joe.Alewine@ibm.com>

Author: joe-alewine