Added details about PYTEST_VERSION to doc/en/reference/reference.rst

Author: dheerajck

doc/en/reference/reference.rst

@@ -1117,6 +1117,11 @@ When set (regardless of value), pytest acknowledges that is running in a CI proc
 This contains a command-line (parsed by the py:mod:`shlex` module) that will be **prepended** to the command line given
 by the user, see :ref:`adding default options` for more information.
 
+.. envvar:: PYTEST_VERSION
+
+This environment variable is defined at the start of the pytest session and is undefined afterwards.
+It contains the value of ``pytest.__version__``, and among other things can be used to easily check if a code is running from within a pytest run.
+
 .. envvar:: PYTEST_CURRENT_TEST
 
 This is not meant to be set by users, but is set by pytest internally with the name of the current test so other