Add an example with custom middleware

examples/markdown_middleware/README.md

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+Cowboy Middleware
+=================
+
+To compile this example you need rebar in your PATH.
+
+Type the following command:
+```
+$ rebar get-deps compile
+```
+
+You can then start the Erlang node with the following command:
+```
+./start.sh
+```
+
+Cowboy will serve all the files you put in the priv/ directory. If you request
+a .html file that has corresponding .md file that has been modified more
+recently than the .html file, the markdown file will be converted to HTML and
+served by Cowboy.
+
+HTML5 Video Example
+-------------------
+
+Open http://localhost:8080/video.html in your favorite browser.