Added set modifier example.

examples/modifiers/set.rb

@@ -0,0 +1,25 @@
+$LOAD_PATH.unshift(File.expand_path('../../../lib', __FILE__))
+require 'mongo_mapper'
+require 'pp'
+
+MongoMapper.database = 'testing'
+
+class User
+  include MongoMapper::Document
+
+  key :name, String
+  key :tags, Array
+end
+User.collection.remove # empties collection
+
+john = User.create(:name => 'John',  :tags => %w[ruby mongo], :age => 28)
+bill = User.create(:name => 'Bill',  :tags => %w[ruby mongo], :age => 30)
+
+User.set({:name => 'John'}, :tags => %[ruby])
+pp john.reload
+
+User.set(john.id, :tags => %w[ruby mongo])
+pp john.reload
+
+john.set(:tags => %w[something different])
+pp john.reload