Add mention message for authors to categorize their own PRs

Author: yaahc

highfive/configs/rust-lang/rust.json

@@ -193,6 +193,10 @@
         "src/tools/miri": {
             "message": "Some changes occured to the Miri submodule",
             "reviewers": ["@rust-lang/miri"]
+        },
+        "library": {
+            "message": "Thank you for submitting a new PR for the library teams! If this PR contains a stabilization of a library feature that has not already completed FCP in its tracking issue, introduces new or changes existing unstable library APIs, or changes our public documentation in ways that create new stability guarantees then please comment with `r? rust-lang/libs-api @rustbot label +T-libs-api` to request review from a libs-api team reviewer.",
+            "reviewers": []
         }
     },
     "new_pr_labels": ["S-waiting-on-review"]