add basic branch protection docs (#569)

* add basic branch protection docs

* example params for branch protection

Author: bobeagan

doc/README.md

@@ -32,6 +32,7 @@ APIs:
 * [Repositories](repos.md)
   * [Contents](repo/contents.md)
   * [Deployments](repo/deployments.md)
+  * [Protection](repo/protection.md)
   * [Releases](repo/releases.md)
     * [Assets](repo/assets.md)
   * [Stargazers](repo/stargazers.md)

doc/repo/protection.md

@@ -0,0 +1,35 @@
+## Repo / Protection API
+[Back to the "Repos API"](../repos.md) | [Back to the navigation](../README.md)
+
+The Protection API is currently available for developers to preview.
+To access the API during the preview period, you must provide a custom media type in the Accept header:
+
+```php
+$client->api('repo')->protection()->configure();
+```
+
+### List all branch protection
+
+> Requires [authentication](../security.md).
+
+```php
+$protection = $client->api('repo')->protection()->show('twbs', 'bootstrap', 'master');
+```
+
+### Update branch protection
+
+> Requires [authentication](../security.md).
+
+For the full list of parameters see https://developer.github.com/v3/repos/branches/#parameters-1
+
+```php
+$params = [
+    'required_status_checks' => null,
+    'required_pull_request_reviews' => [
+        'include_admins' => true,
+    ],
+    'enforce_admins' => true,
+    'restrictions' => null,
+];
+$protection = $client->api('repo')->protection()->show('twbs', 'bootstrap', 'master', $params);
+```