Added ab cheatsheet

cheatsheets/ab

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+# send 100 requests with a concurency of 50 requests to an URL
+ab -n 100 -c 50 http://www.example.com/
+
+# send requests during 30 seconds with a concurency of 50 requests to an URL
+ab -t 30 -c 50 URL http://www.example.com/