ReadMe.rst: Add libspdm submodule license

This patch add libspdm submodule license.

Cc: Andrew Fish <afish@apple.com>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Wenxing Hou <wenxing.hou@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
1 file changed