Move English documentation to folder en_US

doc/en_US/installation.markdown

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+Installation
+============
+
+Firstly, check the [requirements](requirements.markdown) before to go further.
+
+From the archive (stable version)
+---------------------------------
+
+1. You must have a web server with PHP installed
+2. Download the source code and copy the directory `kanboard` where you want
+3. Check if the directory `data` is writeable by the web server user
+4. With your browser go to <http://yourpersonalserver/kanboard>
+5. The default login and password is **admin/admin**
+6. Start to use the software
+7. Don't forget to change your password!
+
+The `data` folder is used to store:
+
+- Sqlite database: `db.sqlite`
+- Debug file: `debug.log` (if debug mode enabled)
+- Uploaded files: `files/*`
+- Image thumbnails: `files/thumbnails/*`
+
+People who are using a remote database (Mysql/Postgresql) and a remote object storage (Aws S3 or similar) don't necessarily need to have a persistent local data folder or to change the permission.
+
+From the git repository (development version)
+---------------------------------------------
+
+You must install [composer](https://getcomposer.org/) to use this method.
+
+1. `git clone https://github.com/kanboard/kanboard.git`
+2. `composer install --no-dev`
+3. Go to the third step just above
+
+Note: This method will install the **current development version**, use at your own risk.
+
+Installation outside of the document root
+-----------------------------------------
+
+If you would like to install Kanboard outside of the web server document root, you need to create at least these symlinks:
+
+```bash
+.
+├── assets -> ../kanboard/assets
+├── cli -> ../kanboard/cli
+├── doc -> ../kanboard/doc
+├── favicon.ico -> ../kanboard/favicon.ico
+├── index.php -> ../kanboard/index.php
+├── jsonrpc.php -> ../kanboard/jsonrpc.php
+└── robots.txt -> ../kanboard/robots.txt
+```
+
+The `.htaccess` is optional because its content can be included directly in the Apache configuration.
+
+You can also define a custom location for the plugins and files folders by changing the [config file](config.markdown).
+
+
+Other Database Types
+--------------------
+
+Kanboard supports Mysql and Postgres as alternative to Sqlite.
+
+- [Mysql configuration](mysql-configuration.markdown)
+- [Postgres configuration](postgresql-configuration.markdown)
+
+Optional Installation
+---------------------
+
+- Some features of Kanboard require that you run [a daily background job](cronjob.markdown) (Reports and analytics)
+- [Install the background worker](worker.markdown) to improve the performances
+
+Security
+--------
+
+- Don't forget to change the default user/password
+- Don't allow everybody to access to the directory `data` from the URL. There is already a `.htaccess` for Apache and a `web.config` file for IIS but nothing for other web servers.