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>

Readme.md

@@ -56,6 +56,7 @@ ARM                 | arm-linux-gnueabihf-
 IA32                | i?86-linux-gnu-* _or_ x86_64-linux-gnu-
 IPF                 | ia64-linux-gnu
 X64                 | x86_64-linux-gnu-
+RISCV64             | riscv64-unknown-elf-
 
 \* i386, i486, i586 or i686
 
@@ -66,6 +67,11 @@ and [arm-linux-gnueabihf](https://releases.linaro.org/components/toolchain/binar
 compiled to run on x86_64/i686 Linux and i686 Windows. Some Linux distributions
 provide their own packaged cross-toolchains.
 
+### GCC for RISC-V
+RISC-V open source community provides GCC toolchains for
+[riscv64-unknown-elf](https://github.com/riscv/riscv-gnu-toolchain)
+compiled to run on x86 Linux.
+
 ### clang
 Clang does not require separate cross compilers, but it does need a
 target-specific binutils. These are included with any prepackaged GCC toolchain
@@ -255,6 +261,12 @@ For more information, see the
 * [Pi 3](Platform/RaspberryPi/RPi3)
 * [Pi 4](Platform/RaspberryPi/RPi4)
 
+## RISC-V
+### SiFive
+* [Sifive U5 Series](Platform/SiFive/U5SeriesPkg) Refer to Platform/SiFive/U5Series/Readme.md on edk2-platform repository.
+* [Freedom U500 VC707 FPGA](Platform/SiFive/U5SeriesPkg/FreedomU500VC707Board)
+* [Freedom U540 HiFive Unleashed](Platform/SiFive/U5SeriesPkg/FreedomU540HiFiveUnleashedBoard)
+
 ## Socionext
 * [SynQuacer](Platform/Socionext/DeveloperBox)