add: yslow-rules showcase

docs/content/showcase/yslow-rules.md

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+title = "YSlow Rules"
+date = 2014-04-07T10:45:00Z 
+description = "Community project of YSlow rules translations"
+sourceLink = "https://github.com/checkmyws/yslow-rules"
+license = "MIT License"
+licenseLink = "https://raw.github.com/checkmyws/yslow-rules/master/LICENSE"
+sitelink = "http://checkmyws.github.io/yslow-rules/"
+thumbnail = "/static/img/yslow-rules.jpg"
+tags = ["community", "documentation", "translation"]
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+