Make the README more clearly.

Signed-off-by: Zhihai Dong <dongzhihai@eswincomputing.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>