[direnv] Post-install hook instructions for direnv

After installing direnv with homebrew you’ll need to run a command to hook direnv into your shell. This information is not present in the current direnv recipe which could make people get confused because direnv doesn’t work “out of the box”.
The command that needs to be run to finish the installation can be found at https://direnv.net/docs/hook.md and varies depending on the shell you use (i.e. bash, zsh, fish, etc).

The instructions look like this:

# Setup

For direnv to work properly it needs to be hooked into the shell. Each shell
has its own extension mechanism.

Once the hook is configured, restart your shell for direnv to be activated.

## BASH

Add the following line at the end of the `~/.bashrc` file:

```sh
eval "$(direnv hook bash)"
```

Make sure it appears even after rvm, git-prompt and other shell extensions
that manipulate the prompt.

## ZSH

Add the following line at the end of the `~/.zshrc` file:

```sh
eval "$(direnv hook zsh)"
```

## FISH

Add the following line at the end of the `~/.config/fish/config.fish` file:

```fish
direnv hook fish | source
```

I created a PR adding a message to the caveats section pointing to the instructions page from direnv but it was rejected because

Read the manual isn’t really a homebrew specific instruction

Should we simply copy and paste the content of the instruction page to the caveats?
Is there another section where we can include this information?

No, all of this information applies to all systems where you install direnv. The caveats section is only for comments about Homebrew deviating from standards or expected behaviour. See https://docs.brew.sh/Formula-Cookbook#caveats