Add testdir examples to CONTRIBUTING guide

[Tadaboody]

Hopefully Closes: #4151

Thanks for submitting a PR, your contribution is really appreciated!

Here's a quick checklist that should be present in PRs (you can delete this text from the final description, this is
just a guideline):

• Create a new changelog file in the changelog folder, with a name like <ISSUE NUMBER>.<TYPE>.rst. See changelog/README.rst for details.
• Target the master branch for bug fixes, documentation updates and trivial changes.
• Target the features branch for new features and removals/deprecations.
• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.

Unless your change is trivial or a small documentation fix (e.g., a typo or reword of a small section) please:

• Add yourself to AUTHORS in alphabetical order;

[codecov]

Codecov Report

Merging #4155 into master will decrease coverage by 1.91%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4155      +/-   ##
==========================================
- Coverage    95.8%   93.88%   -1.92%     
==========================================
  Files         109      109              
  Lines       23828    23823       -5     
  Branches     2352     2349       -3     
==========================================
- Hits        22829    22367     -462     
- Misses        708     1143     +435     
- Partials      291      313      +22

Flag	Coverage Δ
#docs	?
#doctesting	?
#linting	?
#linux	93.77% <ø> (-1.89%)	⬇️
#nobyte	?
#numpy	?
#pexpect	?
#py27	91.8% <ø> (-2.22%)	⬇️
#py34	92.23% <ø> (-0.09%)	⬇️
#py35	?
#py36	92.22% <ø> (-1.84%)	⬇️
#py37	?
#trial	?
#windows	91.1% <ø> (-2.93%)	⬇️
#xdist	?

Impacted Files	Coverage Δ
testing/test_pdb.py	43.31% <0%> (-51.61%)	⬇️
src/_pytest/debugging.py	60.46% <0%> (-34.11%)	⬇️
src/_pytest/unittest.py	74.11% <0%> (-20%)	⬇️
testing/python/approx.py	79.76% <0%> (-19.85%)	⬇️
testing/test_unittest.py	85.18% <0%> (-13.89%)	⬇️
src/_pytest/reports.py	86.9% <0%> (-10.72%)	⬇️
src/_pytest/python_api.py	87.01% <0%> (-10.39%)	⬇️
testing/test_terminal.py	96.65% <0%> (-3.35%)	⬇️
src/_pytest/doctest.py	94.37% <0%> (-2.41%)	⬇️
testing/python/integration.py	89.36% <0%> (-2.13%)	⬇️
... and 14 more

---

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[coveralls]

Coverage increased (+0.05%) to 94.04% when pulling 99d957b on Tadaboody:Add_a_simple_example_on_how_to_use_pytester_to_the_CONTRIBUTING_guide_4151 into 49defa2 on pytest-dev:master.

[nicoddemus]

Awesome, thanks @Tadaboody!

[nicoddemus]

(Merging as the relevant job has already passed 👍)