Make bors ignore the PR template so it doesn't end up in merge messages

Author: joshtriplett

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,3 +1,4 @@
+<!-- homu-ignore:start -->
 <!--
 Thanks for submitting a pull request 🎉! Here are some tips for you:
 
@@ -34,3 +35,4 @@ https://doc.crates.io/contrib/tests
 Other information you want to mention in this PR, such as prior arts,
 future extensions, an unresolved problem, or a TODO list.
 -->
+<!-- homu-ignore:end -->