Add note about ab's performance

Author: kenjis

README.md

@@ -72,6 +72,8 @@ These are my benchmarks, not yours. **I encourage you to run on your (production
 
 Note(1): All the results are run on php with `phalcon.so` and `ice.so`. If you don't load phalcon.so or ice.so, the rps except for Phalcon or Ice probably increase a bit.
 
+Note(2): This benchmarks are limited by `ab` performance. See <https://github.com/kenjis/php-framework-benchmark/issues/62>.
+
 ## How to Benchmark
 
 Install source code as <http://localhost/php-framework-benchmark/>: