Add example for immutable vs mutable strings

Author: avellable

code/string/mutable_vs_immutable_strings.rb

@@ -0,0 +1,12 @@
+require 'benchmark/ips'
+
+IMMUTABLE_TEST = "writing_fast_ruby".freeze
+mutable_test = "writing_fast_ruby"
+
+hash = {"writing_fast_ruby" => "is_cool"}
+
+Benchmark.ips do |x|
+  x.report("freeze") { hash[IMMUTABLE_TEST] }
+  x.report("normal") { hash[mutable_test] }
+  x.compare!
+end