Improve README

- Mention `plug_graphql`
- More appropriate Usage instructions

Author: Josh Price

README.md

@@ -5,6 +5,13 @@
 
 An Elixir implementation of Facebook's GraphQL.
 
+This is the core GraphQL query parsing and execution engine whose goal is to be
+transport, server and datastore agnostic.
+
+In order to setup an HTTP server (ie Phoenix) to handle GraphQL queries you will need (plug_graphql)[https://github.com/joshprice/plug_graphql].
+
+Other ways of handling queries will be added in due course.
+
 ## Installation
 
 First, add GraphQL to your `mix.exs` dependencies:
@@ -23,21 +30,35 @@ $ mix deps.get
 
 ## Usage
 
-Parse a simple GraphQL query
+First setup your schema
+
+```elixir
+defmodule TestSchema do
+  def schema do
+    %GraphQL.Schema{
+      query: %GraphQL.ObjectType{
+        name: "RootQueryType",
+        fields: [
+          %GraphQL.FieldDefinition{
+            name: "greeting",
+            type: "String",
+            resolve: &greeting/1,
+          }
+        ]
+      }
+    }
+  end
+
+  def greeting(name: name), do: "Hello, #{name}!"
+  def greeting(_), do: greeting(name: "world")
+end
+```
+
+Execute a simple GraphQL query
 
 ```elixir
-iex> GraphQL.parse "{ hello }"
-{:ok, %{definitions: [
-  %{kind: :OperationDefinition, loc: %{start: 0},
-    operation: :query,
-    selectionSet: %{kind: :SelectionSet, loc: %{start: 0},
-      selections: [
-        %{kind: :Field, loc: %{start: 0}, name: "hello"}
-      ]
-    }}
-  ],
-  kind: :Document, loc: %{start: 0}
-}}
+iex> GraphQL.execute(TestSchema.schema, "{greeting}")
+{:ok, %{greeting: "Hello, world!"}}
 ```
 
 ## Status