docs: Update release notes for HKDF fix

Add missing release notes from merged fix.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/docs/release-notes.d/hkdf-mem-leak.md b/docs/release-notes.d/hkdf-mem-leak.md
new file mode 100644
index 0000000..b2b8ab6
--- /dev/null
+++ b/docs/release-notes.d/hkdf-mem-leak.md
@@ -0,0 +1 @@
+- fix a memory leak in the HKDF implementation.