feat(traits): add hint comments

exercises/traits/traits1.rs

@@ -7,6 +7,7 @@
 // The trait AppendBar has only one function,
 // which appends "Bar" to any object
 // implementing this trait.
+// Execute `rustlings hint traits1` or use the `hint` watch subcommand for a hint.
 
 // I AM NOT DONE
 

exercises/traits/traits2.rs

@@ -9,6 +9,7 @@
 //
 // No boiler plate code this time,
 // you can do this!
+// Execute `rustlings hint traits2` or use the `hint` watch subcommand for a hint.
 
 // I AM NOT DONE