add require lines for arg/block types in getInfo intro snippet

docs/extensions.md

@@ -75,6 +75,10 @@ All extensions must define a function called `getInfo` which returns an object t
 render both the blocks and the extension itself.
 
 ```js
+// Core, Team, and Official extensions can `require` VM code:
+const ArgumentType = require('../../extension-support/argument-type');
+const BlockType = require('../../extension-support/block-type');
+
 class SomeBlocks {
     // ...
     getInfo () {