Update performance instructions in ReadMe.md Clarified instructions on using AVX and SSE for performance improvements, and added details about compiler flags and FMA instructions.

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 To take advantage of this file just include it in your project that uses ARM NEON intinsics instead of "arm_neon.h", compile it as usual and enjoy the result.
 
-Considering 64-128 bit width of the NEON istructions AVX usage in x86 implementaion is litited, SSE is used in more than 90% of functions.
-For significant performance improvement in some cases you might need to define USE_SSE4 or USE_AVX2 in your project settings. Otherwise by default SIMD up to SSSE3 to be used.
+Considering 64-128 bit width of the NEON istructions AVX vector instructions usage in x86 implementaion is litited, SSE is used in more than 90% of functions.
+However for some functions significant performance improvement it is recommended to compile your x86 code  with the highest level of vector instructions supported by the target CPU -say  SSE4 or AVX2.
+Please notice that AVX2 enabling for this project compilation includes FMA instructions as well.
+Just use the corresponding compiler flags for the purpose. In some cases you might need to define USE_SSE4 or USE_AVX2 in your project settings manually. Otherwise by default SIMD up to SSSE3 to be used.
 
 If NEON2SSE_DISABLE_PERFORMANCE_WARNING macro is defined, then the performance warnings for serial functions implementaions are disabled.