Update books-ja.md / add JavaScript Primer (#3661)

The content of book released under the CC BY-NC 4.0.

free-programming-books-ja.md

@@ -370,6 +370,7 @@
 * [Google JavaScript スタイルガイド](https://w.atwiki.jp/aias-jsstyleguide2/) - Aaron Whyte, Bob Jervis, Dan Pupius, Erik Arvidsson, Fritz Schneider, Robby Walker, aiaswood(翻訳)
 * [JavaScript Garden](http://bonsaiden.github.io/JavaScript-Garden/ja/) - Ivo Wetzel, HIRAKI Satoru(翻訳)
 * [JavaScript Plugin Architecture](https://azu.gitbooks.io/javascript-plugin-architecture/content/) - azu
+* [JavaScript Primer](https://jsprimer.net) - azu, Suguru Inatomi
 * [JavaScript Promiseの本](https://azu.github.io/promises-book/) - azu
 * [JavaScript style guide](https://developer.mozilla.org/ja/docs/JavaScript_style_guide) - MDN
 * [JavaScript 基礎文法最速マスター](http://gifnksm.hatenablog.jp/entry/20100131/1264934942) - id:gifnksm