Update basics.rst

Add the description of `PIPENV_SHELL`

docs/basics.rst

@@ -326,7 +326,7 @@ You should do this for your shell too, in your ``~/.profile`` or ``~/.bashrc`` o
 
 .. note:: The shell launched in interactive mode. This means that if your shell reads its configuration from a specific file for interactive mode (e.g. bash by default looks for a ``~/.bashrc`` configuration file for interactive mode), then you'll need to modify (or create) this file.
 
-Also ``pipenv shell`` use ``$SHELL`` environment variable to get the name of shell you are running, if something wrong, just  check it.
+If you get something wrong with ``$ pipenv shell``, just check ``PIPENV_SHELL`` environment variable, ``$ pipenv shell`` will use it if available. For detail, see `Configuration With Environment Variables <https://docs.pipenv.org/advanced/#configuration-with-environment-variables>`_.
 
 ☤ A Note about VCS Dependencies
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