virtualenvwrapper: add README (#8226)

plugins/virtualenvwrapper/README.md

+# Virtualenvwrapper plugin
+
+This plugin loads Python's [virtualenvwrapper](https://virtualenvwrapper.readthedocs.io/en/latest/) shell tools.
+
+To use it, add `virtualenvwrapper` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... virtualenvwrapper)
+```
+
+## Usage
+
+The plugin allows to automatically activate virtualenvs on cd into git repositories with a matching name:
+
+```
+➜  github $ cd ansible
+(ansible) ➜  ansible git:(devel) $ cd docs
+(ansible) ➜  docs git:(devel) $ cd ..
+(ansible) ➜  ansible git:(devel) $ cd ..
+➜  github $
+```
+
+We can override this by having a `.venv` file in the directory containing a differently named virtualenv:
+
+```
+➜  github $ cat ansible/.venv
+myvirtualenv
+➜  github $ cd ansible
+(myvirtualenv) ➜  ansible git:(devel) $ cd ..
+➜  github $
+```
+
+We can disable this behaviour by setting `DISABLE_VENV_CD=1` before Oh My Zsh is sourced:
+```zsh
+DISABLE_VENV_CD=1
+plugins=(... virtualenvwrapper)
+source $ZSH/oh-my-zsh.sh
+```