Update pipenv install instructions

Follow these steps at the project root:

First, ensure that `~/.local/bin` is in your `PATH` with:

```bash

pip install pipenv # if you don't have pipenv already installed on your system

pipenv install

pipenv shell

> echo $PATH

```

In case `~/.local/bin` is not part of your `PATH` variable, under Linux add it with:

```bash

export PATH=~/.local/bin:$PATH

```

or the equivalent in your particular OS.

Then proceed to install pipenv and the required environment packages:

```bash

> pip install pipenv # if you don't have pipenv already installed on your system

> pipenv install

> pipenv shell

```

You will now be inside a pipenv shell with your virtual environment ready.