Add documentation for task imports using a CSV file and update related sections

[macofmaine]

There was limited documentation on the import of Kanboard tasks using a csv file. So with a little poking around and some testing came up with a little cheat sheet in markdown so you can easily import Kanboard tasks from a csv file.

[fguillot]

The new Markdown document seems to be incorrect.

The column fields are listed here: https://github.com/kanboard/kanboard/blob/f0f7d27007f5a788f14416c548421efd740b7bce/app/Import/TaskImport.php#L34-L56

    public function getColumnMapping()
    {
        return array(
            'reference'         => e('Reference'),
            'title'             => e('Title'),
            'description'       => e('Description'),
            'assignee'          => e('Assignee Username'),
            'creator'           => e('Creator Username'),
            'color'             => e('Color Name'),
            'column'            => e('Column Name'),
            'category'          => e('Category Name'),
            'swimlane'          => e('Swimlane Name'),
            'score'             => e('Complexity'),
            'time_estimated'    => e('Time Estimated'),
            'time_spent'        => e('Time Spent'),
            'date_started'      => e('Start Date'),
            'date_due'          => e('Due Date'),
            'priority'          => e('Priority'),
            'is_active'         => e('Status'),
            'tags'              => e('Tags'),
            'external_link'     => e('External Link'),
        );
    }

Users can also download the task template from the web ui:

The CSV template looks like this:

Reference,Title,Description,"Assignee Username","Creator Username","Color Name","Column Name","Category Name","Swimlane Name",Complexity,"Time Estimated","Time Spent","Start Date","Due Date",Priority,Status,Tags,"External Link"