Write guide demonstrating how the build script can be customised

[michaelsauter]

The ods-build-* tasks are quite opinionated. It would be great to have a guide that walks through an example of replacing the opinionated build logic with a script file from the repository.

FYI @adyachok based on our discussion recently

[michaelsauter]

Closing as a consequence of #722. It is the responsibility of each task repository to maintain the documentation for build tasks.