Suggest stable-nominated in the PR template

Author: tgross35

.github/PULL_REQUEST_TEMPLATE.md

@@ -37,3 +37,9 @@ or mark it as a draft if you are not sure. -->
   included (see [#3131](https://github.com/rust-lang/libc/issues/3131))
 - [ ] Tested locally (`cd libc-test && cargo test --target mytarget`);
   especially relevant for platforms that may not be checked in CI
+
+<!-- labels: is this PR a breaking change? If not, we can probably get it in a
+0.2 release. Just uncomment the following:
+
+@rustbot label +stable-nominated
+-->