Add develop instructions to CONTRIBUTING

From: https://github.com/pytest-dev/pytest/pull/6244

.gitignore

@@ -31,6 +31,7 @@ dist/
 issue/
 env/
 .env/
+.venv/
 3rdparty/
 .tox
 .cache

CONTRIBUTING.rst

@@ -262,6 +262,19 @@ Here is a simple overview, with pytest-specific bits:
 
    When committing, ``pre-commit`` will re-format the files if necessary.
 
+#. If instead of using ``tox`` you prefer to run the tests directly, then we suggest to create a virtual environment and use
+   an editable install with the ``testing`` extra::
+
+       $ python3 -m venv .venv
+       $ source .venv/bin/activate  # Linux
+       $ .venv/Scripts/activate.bat  # Windows
+       $ pip install -e ".[testing]"
+
+   Afterwards, you can edit the files and run pytest normally::
+
+       $ pytest testing/test_config.py
+
+
 #. Commit and push once your tests pass and you are happy with your change(s)::
 
     $ git commit -a -m "<commit message>"