Explanation of stock reports dates

Marking this under the beta2 release plan #2425267: [META] 4.0 Beta2 Release Plan

There should be a hook_help() or README for this, but quick breakdown:

      // Compute the number of weeks since start.
      $now = new DateTime();

      // Check if that product is more recent than the history period.
      if ($start < $product_wrapper->created->value()) {
        $start_datetime = new DateTime();
        $start_datetime->setTimestamp($product_wrapper->created->value());
      }
      else {
        $start_datetime = new DateTime(variable_get('commerce_reports_stock_historyperiod', '3 months ago'));
      }

The date is from the product's creation, or the defined history period, which defaults to -3 months.

Added a README to the module, also put that content here: https://www.drupal.org/node/2334035