docs: checklists: Capture no-unused-structs in changes checklist

Unused structs become an API headache - dependent applications can come
to rely on them despite them having no use by the library.

Provide advice to avoid them altogether.

Change-Id: Ic68d725e5f7b569e77f429e858a69085cde1fb2e
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/docs/checklists/changes.md b/docs/checklists/changes.md
index 5cacf3a..69d2372 100644
--- a/docs/checklists/changes.md
+++ b/docs/checklists/changes.md
@@ -103,6 +103,9 @@
     use pointers for out-parameters, where it has often become ambiguous whether
     the underlying memory represents a single object or an array.
 
+- [ ] Each new `struct` I've defined is used in at least one new function I've
+      added to the public API.
+
 - [ ] My new public `struct` definitions are _not_ marked
       `__attribute__((packed))`