If your build commands are so complex that you need a shell script, go write the shell script and then in the README you tell the users what shell script to invoke and any required parameters. The point is that you always will have a README where you say "run this command to build". At its most basic this formatting is exactly as you would format such a README. It also supports more a complex series of commands, but for the 90% there will just be one command in the "How to build" section