Recommend importing gql literal into each file (#118)

* Recommended that gql literals be imported in all files.

* Fixed error and switched to new graphql-tag package

docs/source/apollo-client/core.md

@@ -20,25 +20,31 @@ const query = gql`
 `
 ```
 
-There are two ways to import and use the `gql` tag. You can either register it globally:
+The `gql` tag is a function under the hood, therefore it must be imported wherever it is used:
 
 ```js
-// Register it globally
-import { registerGqlTag } from 'apollo-client/gql';
-registerGqlTag();
+import gql from 'graphql-tag';
 
-// Now, in any part of your app you can use the gql tag
 const query = gql`...`;
 ```
 
-Alternatively, you can import it in every file if you want to be more explicit:
+Alternatively, if you prefer *not* to import the `gql` tag each time you use it, you can register it as a global:
 
 ```js
-import gql from 'apollo-client/gql';
+// In a browser
+import gql from 'graphql-tag';
+window['gql'] = gql;
+
+// In node.js
+import gql from 'graphql-tag';
+global['gql'] = gql;
 
+// Now, in any part of your app you can use the gql tag
 const query = gql`...`;
 ```
 
+**Note:** ES6 imports are hoisted, which may mean that client code using the `gql` tag gets evaluated before the registration of the global. To avoid race conditions, it's best to just import the tag into each file that uses it.
+
 This template literal tag serves two functions:
 
 1. It parses the query string.