add default styles for Devise OTP to apply max width and centering to…

… QR Code; update README and test app for change;

README.md

@@ -58,6 +58,12 @@ Don't forget to migrate:
 
     rake db:migrate
 
+### Default CSS
+
+To use the default CSS for devise-otp, just require the devise-otp.css file as usual in your application.css file (or equivalent):
+
+    *= require devise-otp
+
 ### Custom views
 
 If you want to customise your views, you can use the following generator to eject the default view files:

app/assets/stylesheets/devise-otp.css

@@ -0,0 +1,4 @@
+.qrcode-container {
+  max-width: 300px;
+  margin: 0 auto;
+}

test/dummy/app/assets/stylesheets/application.css

@@ -8,6 +8,7 @@
  * You're free to add application-wide styles to this file and they'll appear at the top of the
  * compiled file, but it's generally better to create a new file per style scope.
  *
+ *= require devise-otp
  *= require_self
  *= require_tree .
  */