Include mypy instructions in CONTRIBUTING.md (#4271)

* reupload

* Include mypy instructions

* delete file

* fixed trailing whitespaces

* options before file path

Co-authored-by: Christian Clauss <cclauss@me.com>

Author: algobytewise

CONTRIBUTING.md

@@ -155,6 +155,8 @@ We want your work to be readable by others; therefore, we encourage you to note
       return a + b
   ```
 
+  Instructions on how to install mypy can be found [here](https://github.com/python/mypy). Please use the command `mypy --ignore-missing-imports .` to test all files or `mypy --ignore-missing-imports path/to/file.py` to test a specific file.
+
 - [__List comprehensions and generators__](https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions) are preferred over the use of `lambda`, `map`, `filter`, `reduce` but the important thing is to demonstrate the power of Python in code that is easy to read and maintain.
 
 - Avoid importing external libraries for basic algorithms. Only use those libraries for complicated algorithms.