add message to update Cargo.toml when x is changed

Author: zephaniahong

triagebot.toml

@@ -468,6 +468,9 @@ This was probably unintentional and should be reverted before this PR is merged.
 If this was intentional then you can ignore this comment.
 """
 
+[mentions."src/tools/x/src/main.rs"]
+message = "`src/tools/x/src/main.rs` was changed. Bump version of Cargo.toml in `src/tools/x` ."
+
 [assign]
 warn_non_default_branch = true
 contributing_url = "https://rustc-dev-guide.rust-lang.org/contributing.html"