Add docs for lazy mainModule (#154)

* Add docs for lazy mainModule

Added info on the option `{ lazy: true }` for api.mainModule to Module docs.
Is there an equivalent option for `api.addFiles` that makes them available for import without eager loading?

I'd also like to propose adding the `api.mainModule` syntax to the [`Package.js`](http://docs.meteor.com/api/packagejs.html) docs, since that's where I think most people look for the Package.js api

* Update modules.md

Author: coagmano

source/packages/modules.md

@@ -212,6 +212,30 @@ Note that the `import` is `from 'meteor/my-modular-package'`, not `from 'my-modu
 
 Finally, since this package is using the new `modules` package, and the package `Npm.depends` on the "moment" npm package, modules within the package can `import moment from 'moment'` on both the client and the server. This is great news, because previous versions of Meteor allowed npm imports only on the server, via `Npm.require`.
 
+### Lazy loading modules from a package
+
+Packages can also specify a *lazy* main module:
+
+```js
+Package.onUse(function (api) {
+  api.mainModule("client.js", "client", { lazy: true });
+});
+```
+
+This means the `client.js` module will not be evaluated during app
+startup unless/until another module imports it, and will not even be
+included in the client bundle if no importing code is found.
+
+To import a method named `exportedPackageMethod`, simply:
+
+```js
+import { exportedPackageMethod } from "meteor/<package name>";
+```
+
+> Note: Packages with `lazy` main modules cannot use `api.export` to export global
+symbols to other packages/apps.  Also, prior to Meteor 1.4.4.2 it is neccessary to explicitly name the file containing the module: `import "meteor/<package name>/client.js"`.
+
+
 ## Local `node_modules`
 
 Before Meteor 1.3, the contents of `node_modules` directories in Meteor application code were completely ignored. When you enable `modules`, those useless `node_modules` directories suddenly become infinitely more useful: