Easy Fix Here add setup instruction : filetype plugin on

So the setup instructions do not include the step add the line below to .vimrc as far as I saw.
This is an important step, the setup presumes that filetype recognition is already enabled.
```
filetype plugin on
```

If filetype plugin on is not set vim will not determine the filetype.
At some point in the ALE user instructions someone needs to address this.

Please add this bit of instruction into the setup so people don't waste their time, debugging.