virtualenv: add README (#7273)

plugins/virtualenv/README.md

+# virtualenv
+
+The plugin displays information of the created virtual container and allows background theming.
+
+To use it, add `virtualenv` to the plugins array of your zshrc file:
+```
+plugins=(... virtualenv)
+```
+
+The plugin creates a `virtualenv_prompt_info` function that you can use in your theme, which displays
+the basename of the current `$VIRTUAL_ENV`. It uses two variables to control how that is shown:
+
+- `ZSH_THEME_VIRTUALENV_PREFIX`: sets the prefix of the VIRTUAL_ENV. Defaults to `[`.
+
+- `ZSH_THEME_VIRTUALENV_SUFFIX`: sets the suffix of the VIRTUAL_ENV. Defaults to `]`.