Providing a Phing build file along with a project is a good way of allowing automation of certain aspects of the project. The only trouble is that users won't know what's in the build file unless they open it or just run it. You could provide documentation along with the build file so that users know what to use the file for, but a better approach is to list out the targets available in a project. This can be done easily by using the -l (lower case L) or list flag, which will just list the available targets in the supplied build file.
Running this on a build file will produce this sort of output from a build file with two targets, one of which is run as a default.