[guide] Use const when not reassigning reference

Author: robinpokorny

README.md

@@ -1588,7 +1588,7 @@ Other Style Guides
  ```javascript
  // bad
 
- let array = [1, 2, 3];
+ const array = [1, 2, 3];
  let num = 1;
  num++;
  --num;
@@ -1605,7 +1605,7 @@ Other Style Guides
 
  // good
 
- let array = [1, 2, 3];
+ const array = [1, 2, 3];
  let num = 1;
  num += 1;
  num -= 1;