Official Resource Pack for Horizon's End.
On the right side of the screen, click the "Version###" (Latest) link under the "Releases" header. Click the "HorizonsEndResourcePack.zip" link on the next screen. Drag and drop the .zip file in your Resource Pack folder. No need to unzip.
It is NOT recommended to click the green "Code" button and click "Download Zip" as extra files are included in the download and the .zip file needs to be unzipped.