Add explanations of __REAL__ and FDV_ARM_MATH.

Author: tmiw

README.md

@@ -243,7 +243,7 @@ Codec 2 requires a hardware Floating Point Unit (FPU) to run in real time.
 Two build options have been added to support building on microcontrollers:
 1. Setting the `cmake` variable MICROCONTROLLER_BUILD disables position independent code (-fPIC is not used).  This was required for the IMRT1052 used in Teensy 4/4.1).
 
-1. On ARM machines, setting the C Flag \_\_EMBEDDED\_\_ and linking with the ARM CMSIS library will improve performance on ARM-based microcontrollers.
+1. On ARM machines, setting the C Flag \_\_EMBEDDED\_\_ and linking with the ARM CMSIS library will improve performance on ARM-based microcontrollers. \_\_REAL\_\_ and FDV\_ARM\_MATH are additional ARM-specific options that can be set to improve performance if required, especially with OFDM modes.
 
 A CMakeLists.txt example for a microcontroller is below: