I took a look in 17w15a files, and found a recipes folder with a bunch of json files, so I decided to see if I could create a new recipe. I copied a existing one and made this
I saved that as flint.json, and created a small resource pack. Sadly it didn't work.
Resource packs are for clients and cannot change server information. Custom recipes would normally be placed with other world-specific data like loot tables and advancements.
However, currently we cannot add custom recipes without modding the jar, but it will eventually be possible.