Add tests and example for `win_probability`

lib/glicko.ex

@@ -21,6 +21,13 @@ defmodule Glicko do
 		iex> Glicko.new_rating(player, [], [system_constant: 0.5])
 		{1.5e3, 200.27141669877065}
 
+	Calculate the probability of a player winning against an opponent.
+
+		iex> player = Player.new_v1
+		iex> opponent = Player.new_v1
+		iex> Glicko.win_probability(player, opponent)
+		0.5
+
 	"""
 
 	alias __MODULE__.{

test/glicko_test.exs

@@ -35,4 +35,16 @@ defmodule GlickoTest do
 
 		assert_in_delta Player.rating_deviation(player), @valid_player_rating_deviation_after_no_results, 1.0e-4
 	end
+
+	test "win probability with same ratings" do
+		assert Glicko.win_probability(Player.new_v1, Player.new_v1) == 0.5
+	end
+
+	test "win probability with better opponent" do
+		assert Glicko.win_probability(Player.new_v1([rating: 1500]), Player.new_v1([rating: 1600])) < 0.5
+	end
+
+	test "win probability with better player" do
+		assert Glicko.win_probability(Player.new_v1([rating: 1600]), Player.new_v1([rating: 1500])) > 0.5
+	end
 end