Add Changelog

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/ChangeLog.d/use_exp_mod_core.txt b/ChangeLog.d/use_exp_mod_core.txt
new file mode 100644
index 0000000..e145335
--- /dev/null
+++ b/ChangeLog.d/use_exp_mod_core.txt
@@ -0,0 +1,6 @@
+Changes
+   * Changed the default MBEDTLS_ECP_WINDOW_SIZE from 2 to 3.
+     The correlation between this define and RSA decryption performance has
+     changed lately due to security hardening.
+     To fix the performance degradation when using default values the
+     window was increased from 2 to 3.