Expand on direnv section

[bishop527]

The direnv section simply refers the reader to direnv.net for Linux installation instructions (by the the site is not linked on the page). Since there are different ways to get direnv working depending on if you're using pyenv, virtualenv, or virtualenvwrapper, it seems reasonable to direct them to the site. However, after struggling with this myself I found the instructions not straightforward, especially for a new user.
Since the direnv section directly follows the virtualenvwrapper section, I suggest updating with the following instructions to get direnv working with virtualenvwrapper:

1. Add the following to ~/.config/direnv/direnvrc

layout_virtualenv {
    local venv_path="$1"
    source ${venv_path}/bin/activate
}
layout_virtualenvwrapper {
    local venv_path="${WORKON_HOME}/$1"
    layout_virtualenv $venv_path
}

2. Add the following to an .envrc file in the root directory of your project
   layout virtualenvwrapper _name_of_virtual_environment_

Now, the virtual environment will activate when you enter the project directory and deactivate when you leave.