Hello SamWD1989,
Unfortunately you can not directly add description to library folder. As a workaround, create a new column in library and then add description to it. But this column will be available for all folder as well as files.
Second way is, create custom content type and add site column to this content type. Follow the steps as mention in below thread:
http://alfordforney.wordpress.com/2012/01/19/sharepoint-2010-add-description-to-folder/
Hope it could help
Hemendra:Yesterday is just a memory,Tomorrow we may never see
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