docs: add a warning before running create-external-cluster-resources.sh
docs: add a warning before running create-external-cluster-resources.sh

If we don't export RGW_POOL_PREFIX firstly, RGW_POOL_PREFIX will be set to
default value, which is "default". Add a warning in document to give user
instructions.

Signed-off-by: Ma Xinjian <d1eaa362fbe20f4d1e94a590b52fa846afcd0e34@cn.fujitsu.com>

rook/rook,rook/rook,rook/rook,rook/nfs,rook/rook,rook/rook,rook/nfs,rook/nfs