message: doc(Overview): Add initial content
diff: title: Overview
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  +*itk.js* combines [Emscripten](http://emscripten.org/) and [ITK](https://www.itk.org/) to enable high-performance spatial analysis in the web browser.
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  +The project provides tools to build C/C++ code to JavaScript (asm.js) and WebAssembly, transfer data efficently in and out of the Emscripten runtime, and asynchronously execute processing pipelines in a background thread.
  