Well, it's written directly on the README front page:

Code:
Install these packages if you don't have them yet.

apt-get install build-essential apt-get install libssl-dev apt-get install libcurl4-openssl-dev
[If you're on x86_64 then you would add :i386 after the package]

Run build.sh    
Your startup commandline for your Call of Duty dedicated server would look something like this. LD_LIBRARY_PATH=$LD_LIBRARY_PATH:. LD_PRELOAD=/path/to/codextended.so ./cod_lnxded