release-notes: Add imgtool changes

Signed-off-by: Bence Balogh <bence.balogh@arm.com>
Change-Id: I48eabb1dc9696ef50d12fc8782616169ba8acc45
diff --git a/docs/release-notes.d/imgtool-getpub-output-file.md b/docs/release-notes.d/imgtool-getpub-output-file.md
new file mode 100644
index 0000000..414a7c9
--- /dev/null
+++ b/docs/release-notes.d/imgtool-getpub-output-file.md
@@ -0,0 +1 @@
+- imgtool's getpub can print the output to a file