Installer script should be clearer about .local/bin

Currently on a fresh install of ubuntu, running the installer script outputs:

```
Installing Stack to: /usr/local/bin/stack...

-------------------------------------------------------------------------------

Stack has been installed to: /usr/local/bin/stack

WARNING: '/home/mgsloan/.local/bin' is not on your PATH.
For best results, please add it to the beginning of PATH in your profile.
```

Two things could be better about the warning:

* It should explain why the user should put `~/.local/bin` on the PATH
* It should explain how to add it to your profile (add to `~/.profile`)

I would just go ahead and implement this, but I'm setting up a fresh installation of my computer and don't have emacs installed yet.