Add link to benchmark PR in README

As suggested by @deniskorobicyn, adding a link from the `README.md`
to the PR, which has more information on performance characteristics.

Author: Drenmi

README.md

@@ -384,6 +384,9 @@ Comparison:
 
 ##### `Array#new` vs `Fixnum#times + map` [code](code/array/array-new-vs-fixnum-times-map.rb)
 
+Typical slowdown is 40-60% depending on the size of the array. See the corresponding [pull request](pull/91) for
+performance characteristics.
+
 ```
 ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-darwin15]
 Calculating -------------------------------------