Add ChangeLog entry for bug
mbed TLS ChangeLog (Sorted per branch, date)
+= mbed TLS x.x.x branch released xxxx-xx-xx
+
+Bugfix
+ * Fixes an issue with MBEDTLS_CHACHAPOLY_C which would not compile if
+ MBEDTLS_ARC4_C and MBEDTLS_CIPHER_NULL_CIPHER weren't also defined. #1890
+
= mbed TLS 2.12.0 branch released 2018-07-25
Security
