edk2-platforms: Revise Readme.md

Mention to update submodule under for edk2-platforms. It is for RISC-V OpenSBI library.

Signed-off-by: Abner Chang <abner.chang@hpe.com>
Co-authored-by: Daniel Schaefer <daniel.schaefer@hpe.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>

Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>