Update library tracking issue template to set S-tracking-unimplemented

This will help people notice the `S-tracking-*` labels, and if the
tracking issue *is* implemented, they can change the label.

Author: joshtriplett

Diff for: .github/ISSUE_TEMPLATE/library_tracking_issue.md

@@ -2,7 +2,7 @@
 name: Library Tracking Issue
 about: A tracking issue for an unstable library feature.
 title: Tracking Issue for XXX
-labels: C-tracking-issue, T-libs-api
+labels: C-tracking-issue, T-libs-api, S-tracking-unimplemented
 ---
 <!--
 Thank you for creating a tracking issue!
@@ -49,6 +49,8 @@ For larger features, more steps might be involved.
 If the feature is changed later, please add those PRs here as well.
 -->
 
+(Remember to update the `S-tracking-*` label when checking boxes.)
+
 - [ ] Implementation: #...
 - [ ] Final comment period (FCP)[^1]
 - [ ] Stabilization PR