Move English documentation to folder en_US

doc/en_US/application-configuration.markdown

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+Application settings
+====================
+
+Some parameters for the application can be changed on the settings page.
+Only administrators can change those settings.
+
+Go to the menu **Settings**, then choose **Application settings** on the left.
+
+![Application settings](screenshots/application-settings.png)
+
+### Application URL
+
+This parameter is used for email notifications.
+The email footer will contain a link to the Kanboard task.
+
+### Language
+
+The application language can be changed at anytime.
+The language will be set for all users.
+
+### Time zone
+
+By default, Kanboard use UTC as time zone, but you can define your own time zone.
+The list contains all supported time zones by your web server.
+
+### Date format
+
+Input format used for date fields, for examples the due date for tasks.
+
+Kanboard offers 4 different formats:
+
+- DD/MM/YYYY
+- MM/DD/YYYY (default)
+- YYYY/MM/DD
+- MM.DD.YYYY
+
+The [ISO 8601](http://en.wikipedia.org/wiki/ISO_8601) format is always accepted (YYYY-MM-DD or YYYY_MM_DD).
+
+### Custom Stylesheet
+
+Write your own CSS to override or improve Kanboard default style.