I was invited to photograph a sporting event last weekend. They waved the entrance fee and I agreed to share my pictures. The host of the event contacted me asking for hi-res versions of some of the photos. I want to give them permission to use them to promote the sport, their upcoming events and the competitors. I do not however want to sign my rights to these photo's away. Do I need to just include a letter stating they have my permission to use them or do I need to find a specific kind of copyrwrite releaase? Thanks, let me know if I need to clarify anything.