Add a Bitdeli badge to README

Author: bitdeli-chef

README.md

@@ -635,3 +635,7 @@ Copyright 2013 Twitter, Inc.
 Copyright 2013 Marcel Duran and other contributors
 
 Licensed under the [MIT License](http://github.com/marcelduran/webpagetest-api/raw/master/LICENSE)
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+[![Bitdeli Badge](https://d2weczhvl823v0.cloudfront.net/marcelduran/webpagetest-api/trend.png)](https://bitdeli.com/free "Bitdeli Badge")
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