Add a bit about the API's skeleton to the API proposal template

.github/ISSUE_TEMPLATE/propose_design.md

@@ -19,6 +19,24 @@ Why are we doing this? What problems does it solve?
 Introduce and explain the new APIs and concepts. How will this proposal be
 implemented? Provide representative and edge-case examples.
 
+Provide a skeleton of the proposed API by writing out types (don't need their
+members or full implementation) as well as function and method signatures
+(again, just the signature, don't need the function body):
+
+```rust
+pub struct Whatever { ... }
+
+impl Whatever {
+    pub fn new(raw: &web_sys::RawWhatever) -> Self { ... }
+    pub fn another(&self) -> Another { ... }
+}
+
+pub struct Another { ... }
+
+// Does X, Y, and Z when dropped.
+impl Drop for Another {}
+```
+
 ## Drawbacks, Rationale, and Alternatives
 
 Does this design have drawbacks? Are there alternative approaches? Why is this