It is unclear to the (not so) uninitiated what all these modules are about.

backends: seems to be be about remote backend
build-config: could mean anything (I thought it was about distribution but there is a separate distribution module)
legacy: got puzzled by this one too until I dug the pom up
modules: generic term
testing: generic term

How about a section like this

The project is split in different modules.

analyzers: Module to depend on to use Apache Solr analyzers. Also facilitate migration from older versions of Lucene.
backends:: Remote backends receiving an indexing job and executing it via different protocols
build-config: Code related artefacts like checkstyle rules. Link explaining how ot use these rules in Eclipse and IntelliJ IDEA.
distribution: Builds the distribution package
documentation: The project documentation
engine: Engine of the project. Most of the beef is here.
infinispan: Backend storing indexes in Infinispan
integrationtest: Integration tests with various technologies like WildFly, Spring. Also include performance tests.
legacy: Old Maven GAV kept for backward compatibility.
modules: Integration with containers like WildFly
orm: Native integration offering Hibernate Search for Hibernate ORM
serialization: Serialization code used by remote backends.
testing: *not sure what's happening here*