docs: add release notes for reset vector update

Add missing release notes for this change.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/docs/release-notes.d/flash-reset.md b/docs/release-notes.d/flash-reset.md
new file mode 100644
index 0000000..dc73dbb
--- /dev/null
+++ b/docs/release-notes.d/flash-reset.md
@@ -0,0 +1,2 @@
+- Use general flash operations to determine the flash reset vector. This
+  improves support a bit for some configurations of external flash.