Update Reviewer_Checklist.md

Author: chahank

doc/guide/Reviewer_Checklist.md

@@ -5,6 +5,7 @@
 * The code should be easily readable (for infos e.g. here [here](https://treyhunner.com/2017/07/craft-your-python-like-poetry/?__s=jf8h91lx6zhl7vv6o9jo))
 * Variable names should be chosen to be clear. Avoid `item, element, var, list` etc... 
 * Avoid as much as possible hard-coded indices for list (no `x = l[0], y = l[1]`) (see also [here](https://treyhunner.com/2018/03/tuple-unpacking-improves-python-code-readability/))
+* Avoid mutable as default values for functions and methods.
 * Use pythonic loops, list comprehensions etc.
 * Make sure the unit test are testing all the relevant parts of the code
 * Check the docstring (is everything clearly explained, are the default values given an clear)