Some documentation is missing

[Sami32]

Like buildDirectory and outputFile.
It would be nice to avoid to have to look at the code IMO.

[Sami32]

<outputFile> is superseed, overrided by <packageName>.
That mean that:

...
<packageName>${project.artifactId}</packageName>
<packageVersion/>
<releaseNumber>1</releaseNumber>
<architecture>any</architecture>
...
<outputFile>${project.name.short}-${project.version}-any.pkg.tar.xz</outputFile>
...

Will give an output file named:
${project.artifactId}-${project.version}-any.pkg.tar.xz

Not sure if that is a bug or a feature.

EDIT:
That could be a feature taking care to automatically lower case to conform to the Arch repository rule, but in that case it is a bug of the <packageName> that doesn't follow the same Arch mandatory rule.