add a README file

add a readme with a short description and a screenshot

README.md

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+# password_policy
+
+This app enables the the admin to define certain rules for passwords, for example the minimum length of a password.
+
+Once the app is enabled you find the "Password Policy" settings in the admin section:
+
+![](https://github.com/nextcloud/screenshots/blob/master/password_policy/password_policy_settings.png)
+
+By default the app enforces a minimum password length of 10 characters and checks every password against the 1.000.000 most common passwords.
+
+Currently the app checks passwords for public link shares and for user passwords if the database backend is used.
+
+You can easily check passwords for your own app by adding following code to your app:
+
+````
+$eventDispatcher = \OC::$server->getEventDispatcher();
+$event = new Symfony\Component\EventDispatcher\GenericEvent($password);
+$eventDispatcher->dispatch('OCP\PasswordPolicy::validate', $event);
+````