Changelog: Add entry for prime test improvement
diff --git a/ChangeLog b/ChangeLog
index 820c26b..fd625a4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,12 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS x.x.x branch released xxxx-xx-xx
+
+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
+     by FIPS-186-4.
+
 = mbed TLS 2.13.1 branch released 2018-09-06
 
 API Changes