Add array tally for ruby

src/dev/ruby/arrays.md

@@ -12,3 +12,10 @@ puts true if arr.uniq.length == 1
 ```ruby
 (subarray & array) == array
 ```
+
+## Compute values occurences
+
+```ruby
+[1, 1, 2, 3, 4, 5, 5, 5, 5].tally
+=> {1=>2, 2=>1, 3=>1, 4=>1, 5=>4}
+```