Indeed, there is no support for the "read more" function on categories description. You will have to go to the menu Display->Views to add it:
First, edit the file listing_img_desc of the view category and change the line:
echo $this->row->category_description;
to:
echo preg_replace('#<hr *id="system-readmore" */>.*#is','',$this->row->category_description);
Second edit the file listing of both product and category views and change the line:
echo JHTML::_('content.prepare',$this->element->category_description);
to:
echo JHTML::_('content.prepare',preg_replace('#<hr *id="system-readmore" */>#i','',$this->element->category_description));
Then, you will be able to use the read more tag <hr id="system-readmore" /> in your categories description so that only the part before is shown on the listing of categories with the item box layout to "image and description" while the full category description will be displayed on listing products and categories where the "show description" option is toggled on.
Please let us know how it goes and we will add that function to next release of HikaShop.