doc: Add RX support in README file

Signed-off-by: Duy Nguyen <[email protected]>

Author: duynguyenxa

README.md

@@ -12,6 +12,7 @@ The toolchains for the following target architectures are supported:
 - MIPS (32-bit and 64-bit)
 - Nios II
 - RISC-V (32-bit and 64-bit; RV32I, RV32E, RV64I)
+- RX
 - SPARC (32-bit and 64-bit; SPARC V8, SPARC V9)
 - x86 (32-bit and 64-bit)
 - Xtensa (sample_controller and other vendor-specific variants)