Added project logo

Author: ericwbailey

README.md

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-# Accessible HTML Content Patterns
+![Accessible HTML Content Patterns](https://raw.githubusercontent.com/ericwbailey/accessible-html-content-patterns/master/logo.png)
+
 A collection of the full [HTML5 Doctor Element Index](http://html5doctor.com/element-index/), minus the `<details>`, `<summary>`, `<command>`, and `<menu>` tags (which have [poor browser support](http://caniuse.com/#search=menu)), as well as common markup patterns for quick reference.