Setting type of computed attribute for schema breaks output

eeb123dfdb contains a test that fails against master; the other commits contain a proposed fix and doc updates.

It seems that the changes for schema output overloaded the behaviour for setting an alternate attribute key that was already there, as changing the code so that it matched the behaviour described in the README broke other tests. 

Here's the local output from the failing test, note the :string => nil.

```
  1) Failure:
SerializerTest#test_attributes_method_with_computed_attribute_with_type [/Users/ali/Work/github/ali-graham/active_model_serializers/test/serializer_test.rb:90]:
--- expected
+++ actual
@@ -1 +1 @@
-{:user_attributes_with_computed=>{:first_name=>"Jose", :last_name=>"Valim", :full_name=>"Jose Valim"}}
+{:user_attributes_with_computed=>{:first_name=>"Jose", :last_name=>"Valim", :string=>nil, :full_name=>"Jose Valim"}}
```

I couldn't think of any way around this that didn't involve changing the ```attributes``` calling conventions, so I chose to preserve what was already in the code and add type as a separate property. The fix I've adopted may not be quite what's needed, but hopefully it gets part of the way there...