Clarify mutation is allowed in high-scores (#1195)

* Clarify mutation is allowed in high-scores

Adding a test to check against mutating the list has been suggested and rejected because this is an introductory core exercise and immutability has not yet been introduced. However, all of the reasonable examples provided do not mutate the list so it is not clear to new mentors whether preserving the original list is required when deciding if they should accept a solution.

* Provided examples of solutions that mutate

Author: Adam Levin

tracks/python/exercises/high-scores/mentoring.md

@@ -48,6 +48,16 @@ Another variation of this solution includes the start at -1 for slicing the resu
         return sorted(scores)[-1:-4:-1]
 ```
 
+Since immutability has not yet been introduced, solutions that mutate the original list (`scores.pop()` or `scores.sort()`) are acceptable. However, take the opportunity to start introducing this concept in your comments.
+```python
+    def latest(scores):
+        return scores.pop()
+
+    def personal_top_three(scores):
+        scores.sort(reverse=True)
+        return scores[:3]
+```
+
 ### Common Suggestions
 
 - Use **`sorted()`** over **`sort()`**. `sorted()` returns a _**copy**_ , `sort()` _**mutates in place**_.