[FAB-8647]: explain limitation of adding new org
[FAB-8647]: explain limitation of adding new org

While adding new organization to the existed Fabric network there is a
limitation which requires to either setup all newly added peer to be
configured as a leaders or leverage dynamic leader election mechanism.
This commit add section to the channel update doc explaining the limitations.

Change-Id: I7ef1cb6804570f4d6764ca490de0a93bcfd979d4
Signed-off-by: Artem Barger <6d127d713cb520aa658cf81ac2373d74ffc1958c@il.ibm.com>

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