The documentation here:
is really good for getting you started a couple of exceptions. Some of the configuration options in *.mojos.xml are not fully listed, including the <requiresDependencyResolution> parameter, needed to get classpaths correct inside of the ant script. If you fail to set that, you get very strange classpaths (just the target/classes directory) with no other sign of error. If you're familiar with Java Mojo development, I suspect this is obvious, but as a developer coming to this and trying to convert a giant chunk of antrun code into a plugin to achieve reuse, I had to find information on this in a completely different place (the Java mojo stuff), which took me quite a bit of time.
I'd recommend adding a brief section on this parameter, with the table of what is in scope copied from the Java mojo section, or at least put a link to that saying "go read this first."
There is no documentation on this issue re: ant mojos anywhere that I can find, including stackoverflow, etc.