Add entry describing the bug fix

Add entry describing the bug fix in `mbedtls_ecdsa_genkey()`.
diff --git a/ChangeLog b/ChangeLog
index be28645..6a783f6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,10 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS x.xx.x branch released xxxx-xx-xx
+
+Bugfix
+   * Fix returning the value 1 when mbedtls_ecdsa_genkey failed.
+
 = mbed TLS 2.16.0 branch released 2018-12-21
 
 Features