Add a note about supported compilers
Add a note about supported compilers

Adapted from the section in Mbed Crypto, with the addition of specific
compiler versions. Only mention Python 3; Python 2 may still work but
it is no longer supported upstream so we will not maintain
compatibility anymore.

Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls