Tell user to make inventory plugin executable

The current version mentions nothing about chmod +x'ing the rax.py file, which means you get a really weird error message when you try to follow it:

```
raise ValueError, "No closing quotation"
```

... from deep inside shlex.py. I'm sure that makes sense if you realized that meant that it was trying to parse `rax.py` as an ini file...
Merge pull request #5889 from lvh/patch-1

Tell user to make inventory plugin executable