Readme.md. Update “Rake tasks” section

Author: andrykonchin

README.md

@@ -856,9 +856,23 @@ end
 
 ## Rake Tasks
 
+There are a few Rake tasks available out of the box:
+
   * `rake dynamoid:create_tables`
   * `rake dynamoid:ping`
 
+In order to use them in non-Rails application they should be required explicitly:
+
+```ruby
+# Rakefile
+
+Rake::Task.define_task(:environment)
+require 'dynamoid/tasks'
+```
+
+The Rake tasks depend on `:environment` task so it should be declared
+as well.
+
 ## Test Environment
 
 In test environment you will most likely want to clean the database between test runs to keep tests completely isolated. This can be achieved like so