{Doc} Added Extension installation section in doc/extensions/README.md (#12061)
{Doc} Added Extension installation section in doc/extensions/README.md (#12061)


yugangw-msft/azure-cli,yugangw-msft/azure-cli,yugangw-msft/azure-cli,yugangw-msft/azure-cli,yugangw-msft/azure-cli,yugangw-msft/azure-cli