Document addition and removal of BFT nodes from channel config

[yacovm]

There is currently no documentation that explains how to edit the channel config to add a new orderer node and remove it.

For BFT, the following things need to be taken care of:

• Add the orderer to the consenter mapping
• Add the orderer's endpoint to the endpoints.
• Adjust the block validation policy accordingly.

Similarly, for removal.

[arkadiPiven]

I will Take it.

[arkadiPiven]

hyperledger/fabric-samples#1047