Add changelog entry

Add changelog entry for bug fix in sample program.

Signed-off-by: Thomas Daubney <thomas.daubney@arm.com>
diff --git a/ChangeLog.d/fix-x25519-program.txt b/ChangeLog.d/fix-x25519-program.txt
new file mode 100644
index 0000000..af60465
--- /dev/null
+++ b/ChangeLog.d/fix-x25519-program.txt
@@ -0,0 +1,4 @@
+Bugfix
+   * Fix a bug in x25519 example program where the removal of
+     MBEDTLS_ECDH_LEGACY_CONTEXT caused the program not to run. Fixes #4901 and
+     #3191.