Add an ACP list item to the library tracking issue template

Most new API has an associated ACP that is useful to reference, but it
doesn't appear anywhere on the template for new tracking issues. Update
this template to include a link to the ACP.

Author: tgross35

.github/ISSUE_TEMPLATE/library_tracking_issue.md

@@ -51,6 +51,7 @@ If the feature is changed later, please add those PRs here as well.
 
 (Remember to update the `S-tracking-*` label when checking boxes.)
 
+- [ ] ACP: rust-lang/libs-team#...
 - [ ] Implementation: #...
 - [ ] Final comment period (FCP)[^1]
 - [ ] Stabilization PR