Add ChangeLog entry

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/ChangeLog.d/alert_reentrant.txt b/ChangeLog.d/alert_reentrant.txt
new file mode 100644
index 0000000..2d1dc60
--- /dev/null
+++ b/ChangeLog.d/alert_reentrant.txt
@@ -0,0 +1,5 @@
+Bugfix
+   * Fix bug in the alert sending function mbedtls_ssl_send_alert_message()
+     potentially leading to corrupted alert messages being sent in case
+     the function needs to be re-called after initially returning
+     MBEDTLS_SSL_WANT_WRITE.