Mention steps to install PIP3 in docs

You may wish to add these two lines to the readme to help noobs like myself and others get up and running a bit faster:

```
sudo apt-get install python3-setuptools
sudo easy_install3 pip
```

I tracked them down after trying with easy_install (2.7 is installed on 14.10) failed to install properly (missing parse library).

For any noobs that did what I did, after installing PIP3, this fixed the juju-git-deploy install:

```
sudo pip3 install --upgrade juju-git-deploy
```

Cheers