Add English README.

README.en.md

@@ -0,0 +1,29 @@
+# OpenToonz
+
+OpenToonz is a 2D animation suite published by 
+[DWANGO](http://dwango.co.jp/english/). It is based on **Toonz Studio
+Ghibli Version**, originally developed in Italy by
+[Digital Video, Inc.](http://www.toonz.com/), and customized by
+[Studio Ghibli](http://www.ghibli.jp/) over many years of production.
+
+## Operating details
+
+Please refer to the OpenToonz site at <https://opentoonz.github.io>.
+
+## Packaged installers
+
+Please download and install OpenToonz from the installers at
+<https://opentoonz.github.io>.
+
+## Building from source
+
+-   [Windows](how_to_build_win.md) (jp)
+-   [OS X](how_to_build_macosx.md) (jp)
+
+## Licensing
+
+-   Files other than the `thirdparty` directory
+    -   Provided under a [modified BSD license](LICENSE.txt).
+-   Files in the `thirdparty` directory
+    -   Follow the licenses described in the README and source code of each
+        third-party component's directory.

README.md

@@ -2,6 +2,8 @@
 
 ## これは何？
 
+- [English README](README.en.md)
+
 2Dアニメーション制作ソフトウェアです。
 イタリアのDigital Video社が開発し、株式会社スタジオジブリのカスタマイズを経て、ジブリ作品の制作に長年使われてきたソフトウェア「Toonz」が基になっています。