2.21.0

#267 Change the description of Doma
#266 Remove descriptions of Doma 3
#265 Polish annotation processing
#264 Improve build script example
#263 Translate resource bundle messages from Japanese into English
#262 Translate javadoc comments from Japanese into English
#261 Change the sphinx langulage option
#260 Translate development.rst
#259 Format with google-java-format 1.6
#258 Translate docs from Japanese into English
#257 Use Sphinx 1.8.2