Issue #2709535 by mikeryan: Add a README

README.txt

+The migrate_plus module extends the core migration system with API enhancements
+and additional functionality, as well as providing practical examples.
+
+Extensions to base API
+======================
+* A Migration configuration entity is provided, enabling persistance of dynamic
+migration configuration.
+* A ConfigEntityDiscovery class is implemented which enables plugin configuration
+to be based on configuration entities. This is fully general - it can be used
+for any configuration entity type, not just migrations.
+* A MigrationConfigEntityPluginManager class and corresponding
+plugin.manager.config_entity_migration service is provided, to enable discovery
+and instantiation of migration plugins based on the Migration configuration
+entity.
+* A MigrationGroup configuration entity is provided, which enables migrations to
+be organized in groups, and to maintain shared configuration in one place.
+* A MigrateEvents::PREPARE_ROW event is provided to dispatch hook_prepare_row()
+invocations as events.
+* A SourcePluginExtension class is provided, enabling one to define fields and
+IDs for a source plugin via configuration rather than requiring PHP code.
+
+Plugins
+=======
+* A Url source plugin is provided, implementing a common structure for
+file-based data providers.
+* XML and JSON fetchers and parsers for the Url source plugin are provided.
+
+Examples
+========
+* The migrate_example submodule provides a fully functional and runnable
+example migration scenario demonstrating the basic concepts and most common
+techniques for SQL-based migrations.
+* The migrate_example_advanced submodule provides examples of migration from
+different kinds of sources, as well as less common techniques.