added a readme with a FAQ

Author: jmslbam

readme.md

@@ -0,0 +1,13 @@
+# Custom 404 page for Clarkson
+ 
+A plugin to set a custom page as 404 when using [Clarkson Core](http://wp-clarkson.com/core){:target="_blank"}
+
+## How to
+
+1. After installing this plugin create a page which it's title and content will be used to show when a 404 is triggered.
+1. Then visit the `Settings > Reading` page within the WordPress Dashboard and select this page from the **404 Page: Dropdown**.
+
+## FAQ
+
+- **I can't see the page content of my select page on a 404.**  
+Please create a `404.twig` template file within your theme.