First draft of xplat migration guide (#960)
First draft of xplat migration guide (#960)

* First draft of xplat migration guide
samedder/azure-cli,samedder/azure-cli,QingChenmsft/azure-cli,QingChenmsft/azure-cli,samedder/azure-cli,QingChenmsft/azure-cli,QingChenmsft/azure-cli,samedder/azure-cli,yugangw-msft/azure-cli,yugangw-msft/azure-cli,yugangw-msft/azure-cli,yugangw-msft/azure-cli,yugangw-msft/azure-cli,yugangw-msft/azure-cli