Move English documentation to folder en_US

doc/en_US/cloudron.markdown

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+How to run Kanboard on Cloudron
+================================
+
+[Cloudron](https://cloudron.io) is a private smartserver on which you can install web
+apps like Kanboard. You can install Kanboard into a custom domain and each
+installation is backed up and kept up-to-date with Kanboard releases automatically.
+
+[![Install](https://cloudron.io/img/button.svg)](https://cloudron.io/button.html?app=net.kanboard.cloudronapp)
+
+Accounts
+--------
+
+The app integrates tightly with the Cloudron User Management (via LDAP). Only
+Cloudron users can access the Kanboard. In addition, any Cloudron administrator
+becomes a Kanboard administrator automatically.
+
+Installing Plugins
+------------------
+
+Plugins can be installed and configured using the [Cloudron CLI](https://git.cloudron.io/cloudron/cloudron-cli)
+tool. See the [app description](https://cloudron.io/appstore.html?app=net.kanboard.cloudronapp) for
+more information.
+
+Application Source code
+----------------------
+
+The source code for the Cloudron app is [here](https://git.cloudron.io/cloudron/kanboard-app).
+