CD Projekt RED has released a new minor patch for the PC version of The Witcher 3: Wild Hunt. This patch is 4GB in size on GOG.com and around 20-100MB on Steam.
From the looks of it, this is a hotfix that introduces a retro-active fix and mod support (do note that the game's Modkit is not included, you have to download it manually from NexusMods).
Patch 1.08.2 (17.08.2015)
Introduces a retro-active fix to some of the achievements that were amended in the patch 1.08.
Adds sources for scripts (mod support).