docs(DEVELOPER.MD): Adds steps for adding new plugins to index.ts (#444)

* Adds steps for updating index.ts

* Adds .DS_Store to gitignore

* Adds step for exporting

.gitignore

@@ -1,3 +1,4 @@
+.DS_Store
 node_modules/
 .idea
 dist/

DEVELOPER.md

@@ -102,6 +102,58 @@ The `@Cordova` decorator has a few more options now.
 
 `clearFunction` is used in conjunction with the `observable` option and indicates the function to be called when the Observable is disposed.
 
+### Updating index.ts
+
+For new plugins, you will need to update `/src/index.ts` to properly export your plugin and make it available for use.
+
+1. Import the plugin class into `index.ts`:
+
+`import {PluginClassName} from ./plugins/filenameForPlugin`
+
+No need to put the `.ts` extension on the filename.
+
+2. Add the plugin class name to the list in the `export` object:
+
+```
+export {
+  ActionSheet,
+  AdMob,
+  AndroidFingerprintAuth,
+  YourPluginClassName,
+  ...
+}
+```
+
+3. Add the plugin class name to the `window['IonicNative']` object:
+
+```
+window['IonicNative'] = {
+  ActionSheet: ActionSheet,
+  AdMob: AdMob,
+  AndroidFingerprintAuth: AndroidFingerprintAuth,
+  YourPluginClassName: YourPluginClassName,
+  ...
+```
+
+4. If your plugin exports any other objects outside of the plugin class, add an export statement for the file:
+
+`export * from './plugins/filenameForPlugin';`
+
+No need to put the `.ts` extension on the filename.
+
+For example, `googlemaps.ts` exports a const outside of the plugin's main `GoogleMap` class:
+
+```
+export const GoogleMapsAnimation = {
+  BOUNCE: 'BOUNCE',
+  DROP: 'DROP'
+};
+```
+
+To properly export `GoogleMapsAnimation`, `index.ts` is updated with:
+
+`export * from './plugins/googlemaps';`
+
 ### Testing your changes
 
 You need to run `npm run build` in the `ionic-native` project, this will create a `dist` directory. Then, you must go to your ionic application folder and replace your current `node_modules/ionic-native/dist/` with the newly generated one.