Add README.md

Author: cboulanger

README.md

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+# Versioned Qooxdoo documentation
+
+This repository contains Qooxdoo's documentation in a versioned
+format. 
+
+Whenever a tag is pushed onto the [main framework
+repository](https://github.com/qooxdoo/qooxdoo), the content of
+the `docs` folder of that tag's source tree is deployed to the
+`gh-pages` branch of this repository in a folder named after the
+tag and published at `https://qooxdoo.org/documentation/<tag>`.
+
+Tags of the form `x.y.z` are shortened to `x.y` so that patch releases do
+not create separate folders. Instead, the folder of the last feature release 
+will be overwritten.