Add ChangeLog for OID-to-string fixes

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/ChangeLog.d/fix-oid-to-string-bugs.txt b/ChangeLog.d/fix-oid-to-string-bugs.txt
new file mode 100644
index 0000000..799f444
--- /dev/null
+++ b/ChangeLog.d/fix-oid-to-string-bugs.txt
@@ -0,0 +1,6 @@
+Bugfix
+   * Fix bug in conversion from OID to string in
+     mbedtls_oid_get_numeric_string(). OIDs such as 2.40.0.25 are now printed
+     correctly.
+   * Reject OIDs with overlong-encoded subidentifiers when converting
+     OID-to-string.