There's no official documentation yet on how to install Railgun daemon on CentOS 8, however you can do that with the following commands:

$ curl -svO http://pkg.cloudflare.com/pool/el7/main/r/railgun-stable-5.3.3-1.el7.src.rpm/railgun-stable-5.3.3-1.el7.x86_64.rpm
$ sudo yum install -y memcached
$ sudo rpm -Uvh railgun-stable-5.3.3-1.el7.x86_64.rpm

Then configure Railgun with your credentials.