Change auto-venv to be opt-in

plugins/python/README.md

@@ -22,7 +22,9 @@ plugins=(... python)
 
 ## Virtual environments
 
-The plugin provides three utilities to manage Python venvs:
+The plugin provides three utilities to manage Python 3.3+
+[venv](https://docs.python.org/3/library/venv.html) virtual
+environments:
 
 - `mkv [name]`: make a new virtual environment called `name` (default: `venv`) in current directory.
 
@@ -30,10 +32,21 @@ The plugin provides three utilities to manage Python venvs:
 
 - `auto_vrun`: Automatically activate the venv virtual environment when
   cd’ing into a directory containing `venv/bin/activate`, and
-  automatically deactivate the venv virtual environment when cd’ing into
-  any other directory.
-  - Set the environment variable `VENV_NAME` to auto-activate on a
-  different venv name. (Example: `VENV_NAME=.venv`).
-  - Set the environment variable `DISABLE_AUTO_VRUN` to anything to
-  deactivate `auto_vrun`. (Example: `DISABLE_AUTO_VRUN=true`) Unset
-  `DISABLE_AUTO_VRUN` to re-enable.
+  automatically deactivate it when cd’ing into any other directory,
+  including subdirectories.
+  - To enable, set `PYTHON_AUTO_VRUN` to anything. For example:
+
+        export PYTHON_AUTO_VRUN='true'
+  - To disable, either unset the environment variable:
+
+        unset PYTHON_AUTO_VRUN
+    or entirely remove `auto_vrun` from the list of functions associated
+    with the `chpwd` hook:
+
+        add-zsh-hook -d chpwd auto_vrun
+    If you disable `auto_vrun` while a virtual environment is active,
+    you'll need to manually `deactivate`.
+  - The default virtual environment name is "venv". To use a different
+    name, set `PYTHON_VENV_NAME`. For example:
+
+        export PYTHON_VENV_NAME='.venv'