added README for extensions.

extensions/README.md

+This folder contains official Yii 2 extensions.
+
+To add a new extension named `xyz` (must be in lower case), take the following steps:
+
+1. create a folder named `xyz` under `yii` and put all relevant source code there;
+2. create the following accessory files (please refer to any existing extension):
+   * `composer.json`
+   * `README.md`
+   * `CHANGELOG.md`
+   * `LICENSE.md`
+3. ask Qiang to create a subsplit for `xyz` and a composer package named `yii2-xyz`;
+4. modify `/composer.json` and add `xyz` to the `replace` section;
+5. modify `/build/controllers/PhpDocController.php` about the method `actionProperty()`.