Add update pip and setuptools section (#4061)

* Add update pip and setuptools section

* Simplify the command to upgrade pip

Co-authored-by: Grey Li <withlihui@gmail.com>

CONTRIBUTING.rst

@@ -112,6 +112,12 @@ First time setup
              > py -3 -m venv env
              > env\Scripts\activate
 
+-   Upgrade pip and setuptools.
+
+    .. code-block:: text
+
+        $ python -m pip install --upgrade pip setuptools
+
 -   Install the development dependencies, then install Flask in editable
     mode.