Merge branch 'master' into refactor-hints

exercises/variables/variables1.rs

@@ -1,6 +1,13 @@
 // variables1.rs
 // Make me compile! Execute the command `rustlings hint variables1` if you want a hint :)
 
+// About this `I AM NOT DONE` thing:
+// We sometimes encourage you to keep trying things on a given exercise,
+// even after you already figured it out. If you got everything working and
+// feel ready for the next exercise, you the `I AM NOT DONE` comment below.
+
+// I AM NOT DONE
+
 fn main() {
     x = 5;
     println!("x has the value {}", x);