This latest release makes Nominatim more configurable in custom installations. There is a new reverse-only mode for imports where forward search is not needed. In addition it is now possible to configure which OSM tags are taken into account by Nominatim. This allows to describe exactly which types of objects should be found in your instance. There are a number of example configurations for frequent use cases which you might use as a starting point. For more information read the installation documentation.
As usual there are also a number of bug fixes and minor improvements. Details can be found in the ChangeLog file.
If you want to update from 3.x.x, there is a database migration path described in the migration guide. Migrating from 3.1.0 can be done without problems. However, when updating from 3.0.x, a full database reimport is strongly recommended. Updating from older releases is only possible with a database reimport.
This will be the last release to support PHP 5.x and PostgreSQL < 9.3.