Changelog: Add entry for prime validation fix

ChangeLog

@@ -2,6 +2,24 @@ mbed TLS ChangeLog (Sorted per branch, date)
 
 = mbed TLS x.x.x branch released xxxx-xx-xx
 
+Security
+   * Fix mbedtls_mpi_is_prime() to use more rounds of probabilistic testing. The
+     previous settings for the number of rounds made it practical for an
+     adversary to construct non-primes that would be erroneously accepted as
+     primes with high probability. This does not have an impact on the
+     security of TLS, but can matter in other contexts with potentially
+     adversarially-chosen numbers that should be prime and can be validated.
+     For example, the number of rounds was enough to securely generate RSA key
+     pairs or Diffie-Hellman parameters, but was insufficient to validate
+     Diffie-Hellman parameters properly.
+     See "Prime and Prejudice" by by Martin R. Albrecht and Jake Massimo and
+     Kenneth G. Paterson and Juraj Somorovsky.
+
+New deprecations
+   * Deprecate the function mbedtls_mpi_is_prime() in favor of
+     mbedtls_mpi_is_prime_ext() which allows specifying the number of
+     Miller-Rabin rounds.
+
 Changes
    * Add MBEDTLS_MPI_GEN_PRIME_FLAG_LOW_ERR flag to mbedtls_mpi_gen_prime() and
      use it to reduce error probability in RSA key generation to levels mandated