RFC for Chef 12 Attr changes

So I just tested this:

```
$ chef-shell -a
...
Session type: standalone
Chef Version: 12.0.0.alpha.1
...
chef > node.default['foo']['bar']['baz'] = 12
 => 12
chef > node.default['foo']['bar'] = {'thing' => 'stuff'}
 => {"thing"=>"stuff"}
chef > node['foo']
 => {"bar"=>{"thing"=>"stuff"}}
```

It's possible my testing is wrong, but if not, we might want to clarify this by making the second command be role_default or similar:

```ruby
chef > node.default['foo']['bar']['baz'] = 12
 => 12
chef > node.role_default['foo']['bar'] = {'thing' => 'stuff'}
 => {"thing"=>"stuff"}
chef > node['foo']
 => {"bar"=>{"baz"=>12, "thing"=>"stuff"}}
```

Btw, this came from my wondering why we didn't have an rm_normal/rm_set specified in the list (rm_normal/rm_set aren't needed because there is only one component for that precedence level).