Remove alignment requirement for mbedtls_gcm_update: documentation

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/ChangeLog.d/gcm-update.txt b/ChangeLog.d/gcm-update.txt
new file mode 100644
index 0000000..4511fec
--- /dev/null
+++ b/ChangeLog.d/gcm-update.txt
@@ -0,0 +1,4 @@
+Features
+   * The multi-part GCM interface (mbedtls_gcm_update() or
+     mbedtls_cipher_update()) no longer requires the size of partial inputs to
+     be a multiple of 16.