Add documentation in english

As some (or many) it@M projects use english as developer language, the README also should contain a section with usage instruction in english.

Example project:

Start by opening the repository README and reviewing its current language and usage sections. Add an English usage-instructions section for the formatter; the work is done when developers can find those instructions in the README and the existing content remains intact.

索引モデルが issue の本文から書いたものです。