docs: Add release note snippet on builtin key support

Signed-off-by: David Vincze <david.vincze@arm.com>
Change-Id: I730a02067c0c51c46189e86fa5ddd0f325311874
diff --git a/docs/release-notes.d/bootutil-builtin-keys.md b/docs/release-notes.d/bootutil-builtin-keys.md
new file mode 100644
index 0000000..d86918b
--- /dev/null
+++ b/docs/release-notes.d/bootutil-builtin-keys.md
@@ -0,0 +1,2 @@
+- Added support for using builtin keys for image validation
+  (available with the PSA Crypto API based crypto backend for ECDSA signatures).