New property ${subproject.subpath} that is the path to the sub-project.

This should be empty for the top-level project or the path (ending in slash) to append to ${project.url}.

book/pom.xml

@@ -36,6 +36,7 @@ along with semanticcms-file-bom.  If not, see <https://www.gnu.org/licenses/>.
     <!-- Must be set to ${git.commit.time} for snapshots or ISO 8601 timestamp for releases. -->
     <project.build.outputTimestamp>${git.commit.time}</project.build.outputTimestamp>
     <module.name>com.semanticcms.file.bom.book</module.name>
+    <subproject.subpath>devel/</subproject.subpath>
     <!-- These values are copied from the project being documented -->
     <documented.artifactId>semanticcms-file-bom</documented.artifactId>
     <documented.name>SemanticCMS File BOM</documented.name>