g:deoplete#sources is deprecated 

The "Install" part of the README is seems to be outdated.

Deoplete logs the message:

```
[deoplete] g:deoplete#sources is deprecated variable.  Please use deoplete#custom#option() instead.
```

I'm on [deoplete.nvim - 2902c37](https://github.com/Shougo/deoplete.nvim/commit/2902c3780aaa497ecf7bfd9d3e53c15f6481afeb), which is origin/master at this time.