add some exercises

exercises/done/cards_of_suit.rb

@@ -0,0 +1,14 @@
+#!/usr/bin/env ruby
+
+def cards_of_suit(cards, suit)
+  cards.select { |card| card.include?(suit) }
+end
+
+cards = [
+  "7 of hearts",
+  "king of clubs",
+  "10 of hearts",
+  "ace of spades"
+]
+
+puts cards_of_suit(cards, "hearts")