Add proposed commit message section to PR Template (MarkBind#796)

Our development workflow now requires contributors to propose a merge
commit message for their PRs before merging.

Let's modify the PR GitHub template to add a section for that, to allow
contributors to fill in their own proposed message.

.github/PULL_REQUEST_TEMPLATE

@@ -37,3 +37,16 @@
 
 
 **Testing instructions:**
+
+
+**Proposed commit message: (wrap lines at 72 characters)**
+
+<!--
+    See this link for more info on how to write a good commit message:
+    https://oss-generic.github.io/process/docs/FormatsAndConventions.html#commit-message
+-->
+
+<!--
+    Some of the responses that you gave to the previous questions might
+    provide you with the information needed to craft your commit message.
+-->