Add description for assignment 13

src/assignments/assignment13.rs

@@ -2,6 +2,7 @@
 //!
 //! The primary goal of this assignment is to get used to data parallelism.
 //!
+//! Refer to your solution for assignment 09. You will implement the parallelized version of assignment 09.
 //! You should fill out the `todo!()` placeholders in such a way that `/scripts/grade-13.sh` works fine.
 //! See `assignment13_grade.rs` and `/scripts/grade-13.sh` for the test script.