Move English documentation to folder en_US

doc/en_US/calendar.markdown

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+Calendar
+========
+
+There are two different views for the calendar:
+
+- The project view with filters (available from the board)
+- The user view (available from the dashboard and from the user section)
+
+At this time the calendar is able to display this information:
+
+- Tasks with a due date, displayed at the top. **The due date can be changed by moving the task to another day**.
+- Tasks based on the creation date or the start date. **These events cannot be modified with the calendar**.
+- Sub-task time tracking, all recorded time slot will be shown in the calendar.
+- Sub-task estimates, forecasts of work left
+
+![Calendar](screenshots/calendar.png)
+
+The calendar configuration can be changed in the settings page.
+
+Note: The due date doesn't contain time information.