Clarify support for mixed-endian platforms

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>

README.md

@@ -261,6 +261,7 @@ Mbed TLS is mostly written in portable C99; however, it has a few platform requi
 - Signed integers must be represented using two's complement.
 - `int` and `size_t` must be at least 32 bits wide.
 - The types `uint8_t`, `uint16_t`, `uint32_t` and their signed equivalents must be available.
+- Mixed-endian platforms are not supported.
 
 PSA cryptography API
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