add more custom module formatter documentation

MODULES.md

@@ -111,6 +111,8 @@ export function bar() {
 
 ## Custom
 
+You can alternatively specify module names instead of one of the built-in types.
+
 ```javascript
 module.exports = ModuleFormatter;
 
@@ -118,6 +120,11 @@ function ModuleFormatter() {
 
 }
 
+ModuleFormatter.prototype.transform = function (ast) {
+  // this is ran after all transformers have had their turn at modifying the ast
+  // feel free to modify this however
+};
+
 ModuleFormatter.prototype.import = function (node, nodes) {
   // node is an ImportDeclaration
 };