Implement README for mdBook Manual

manual/README.md

+# Ion Manual
+
+This manual is meant to be generated via [mdBook](https://github.com/azerupi/mdBook). Simply
+executing `mdbook build` in this directory will build all the HTML/JS/CSS files into the
+**book** directory. Executing `mdbook serve` will have **mdbook** act has a web service
+which can be accessed at http://localhost:3000.
+
+At some point, Ion may provide a native builtin for quick access to reading the documentation
+of the shell, similarly to what Fish does with it's own documentation. This will require
+integrating **mdbook**'s library interface into Ion.