Update failure tests README

[SaitTalhaNisanci]

I keep finding this page when trying to run failure tests, so updating the README that way:
pypa/pipenv#3363 (comment)

[hanefi]

On my dev machine, your changes break the working list of documentation.

❯ pipenv --python 3.6
Creating a virtualenv for this project…
Using /home/hanefi/.pyenv/versions/3.6.12/bin/python3.6m (3.6.12) to create virtualenv…
⠋Running virtualenv with interpreter /home/hanefi/.pyenv/versions/3.6.12/bin/python3.6m
Using base prefix '/home/hanefi/.pyenv/versions/3.6.12'
New python executable in /home/hanefi/.local/share/virtualenvs/temp-fmDL7Ayc/bin/python3.6m
Also creating executable in /home/hanefi/.local/share/virtualenvs/temp-fmDL7Ayc/bin/python
Installing setuptools, pkg_resources, pip, wheel...done.

Virtualenv location: /home/hanefi/.local/share/virtualenvs/temp-fmDL7Ayc
Creating a Pipfile for this project…

❯ pipenv --python 3.6 --python=`which python3`
Virtualenv already exists!
Removing existing virtualenv…
Creating a virtualenv for this project…
Using /home/hanefi/.pyenv/shims/python3 (3.8.5) to create virtualenv…
⠋Running virtualenv with interpreter /home/hanefi/.pyenv/shims/python3
Using base prefix '/home/hanefi/.pyenv/versions/3.8.5'
/usr/lib/python3/dist-packages/virtualenv.py:1090: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
  import imp
New python executable in /home/hanefi/.local/share/virtualenvs/temp-fmDL7Ayc/bin/python3
Also creating executable in /home/hanefi/.local/share/virtualenvs/temp-fmDL7Ayc/bin/python
Installing setuptools, pkg_resources, pip, wheel...done.

Virtualenv location: /home/hanefi/.local/share/virtualenvs/temp-fmDL7Ayc
Warning: Your Pipfile requires python_version 3.6, but you are using 3.8.5 (/home/hanefi/.local/share/v/t/bin/python).
  $ pipenv check will surely fail.

[hanefi]

For more context, I use pyenv to manage multiple python versions.

❯ pyenv versions
  system
  3.6.12
* 3.8.5 (set by /home/hanefi/.pyenv/version)

My Linux distro does not have packages for python3.6, so I started to use pyenv to handle that.

[codecov]

Codecov Report

Merging #5197 (81aa1c5) into master (7e39c7e) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5197      +/-   ##
==========================================
- Coverage   94.20%   94.20%   -0.01%     
==========================================
  Files         214      214              
  Lines       42121    42121              
==========================================
- Hits        39682    39681       -1     
- Misses       2439     2440       +1