I have a skeleton farm and I enchanted DOZENS of items already. I never saw "mending" though. The Wiki says, that you find this on the lower enchanting levels, but I never saw it.
Note: (Normally) Before I enchant an item, I look through all of them and check on each what I would get: Helmet, Chestplate, Leggings, Boots, Sword, Bow. I think chances that I really never could have seen it are pretty low.
Can you get this enchantment only from the end and from books? I saw some enchanted items in the end cities with mending.
Best Answer
Mending is a treasure enchantment, which you can only find in the world, you can not create it yourself on an enchanting table.
Treasure enchantments can be obtained by
Mending can be found on many items, including, for example, Fishing Rods.
Sources: 1.9 changelog, enchantment mechanics, trading pages.