chore: add README to extension example (GoogleCloudPlatform#1652)

Author: bshaffer

appengine/standard/extensions/README.md

@@ -0,0 +1,39 @@
+# Custom Extensions for App Engine Standard
+
+This sample shows how to compile custom extensions for PHP that aren't already included in
+the [activated extensions](https://cloud.google.com/appengine/docs/standard/php-gen2/runtime#enabled_extensions) 
+or [dynamically loadable extensions](https://cloud.google.com/appengine/docs/standard/php-gen2/runtime#dynamically_loadable_extensions).
+
+This can be useful for activating extensions such as [sqlsrv](https://pecl.php.net/package/sqlsrv) which are not (yet) supported
+by this runtime.
+
+## Steps to compiling and activating custom extensions
+
+1. Put the custom extension code in a directory in your project, so it gets uploaded with
+the rest of your application. In this example we use the directory named `ext`.
+
+2. Put the commands to compile the extension and move it into the `vendor` directory
+in your `composer.json`. 
+
+```json
+{
+    "scripts": {
+        "post-autoload-dump": [
+            "cd ext && phpize --clean && phpize && ./configure && make",
+            "cp ext/modules/sqlsrv.so vendor/"
+        ]
+    }
+}
+```
+**NOTE**: Moving the extension into the `vendor` directory ensures the file is cached. This
+means if you modify the ext directory, you'll need to run gcloud app deploy with the
+`--no-cache argument` to rebuild it.
+
+3. Activate the extension in your `php.ini`:
+```ini
+# php.ini
+extension=/workspace/vendor/my_custom_extension.so
+```
+
+4. Deploy your application as usual with `gcloud app deploy`. In this example, we use `index.php`
+to print `phpinfo()` so we can see that the extension has been activated.