edk2-platforms: Update Readme.md for RISC-V platform.

Update Readme.md for RISC-V platforms.

Signed-off-by: Abner Chang <abner.chang@hpe.com>
Reviewed-by: Gilbert Chen <gilbert.chen@hpe.com>
Acked-by: Leif Lindholm <leif.lindholm@linaro.org>

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Gilbert Chen <gilbert.chen@hpe.com>
Cc: Palmer Dabbelt <palmer@sifive.com>