This is copy of Open Territory project. It's not in active development (since 2010), and I am not its author. I simply cloned it to Github for convenience. Original readme file, containing full description and installation instructions is "readme.txt".
P.S. Currently developing successors of Open Territory / XreaL - Daemon Engine, OpenWolf Engine

XreaL is a heavily improved Quake 3 Arena engine. It's bundled with new tools and a demo game that was never finished.
ET:XreaL is a subproject bringing all XreaL enhancements to Return to Castle Wolfenstein: Enemy Territory.
Open Territory is fork of ET:XreaL

Open Territory official page: Sourceforge
Open Territory official website not currently working, but copy of it exists on Web Archive.
Open Territory source code (cloned here), Open Territory etmain folder (cloned here)

ET:XreaL official pages: Sourceforge, Moddb / Old thread on Splashdamage forums
XreaL official website not currently working, but copy of it exists on Web Archive.
ET:XreaL source code (cloned here), XreaL Shader manual (PDF), ETXRadiant, Omni-bot, Texture Guidelines

Full ET:XreaL engine package, including game files (1.5 Gb) you can download from Sourceforge or from Moddb
Wolfenstein: Enemy Territory is free game, you can download it here

Everything else - look in original readme file (readme.txt).