Update README.md

Updates usage example

README.md

@@ -33,6 +33,7 @@ class Resource
   attr_accessor :password
 
   validate :password_length
+  validate :blank_password
 
   def initialize(password)
     @password = password
@@ -43,6 +44,10 @@ class Resource
   def password_length
     warnings.add(:password, "min length should be 5") if password.length < 5
   end
+
+  def blank_password
+    errors.add(:password, 'should not be blank') if password.length == 0
+  end
 end
 
 resource = Resource.new('safe')