Add package-info documentation

data/src/main/java/org/mercury_im/messenger/data/mapping/package-info.java

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+/**
+ * The mapping package contains mapper classes that map database models to entities.
+ * The models can be found in {@link org.mercury_im.messenger.data.model} in this module,
+ * while the entity classes are located in the <pre>entity</pre> module.
+ *
+ * The mapper classes define an architectural boundary that separates the entities from
+ * their database model counterparts. This is done in order to keep the database logic separated
+ * from the domain logic and to allow for quick replacement of the database implementation.
+ *
+ */
+package org.mercury_im.messenger.data.mapping;

data/src/main/java/org/mercury_im/messenger/data/model/package-info.java

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+/**
+ * The model package contains requery model definitions.
+ * All of those classes are abstract, since the requery framework generates concrete implementations
+ * during compilation. Those files can later be found in
+ * <pre>build/generated/sources/annotationProcessor/java/main/...</pre>.
+ *
+ * The structure of the model classes closely mimics the structure of their entity pendants
+ * declared in the <pre>entity</pre> module.
+ *
+ * @see <a href="https://github.com/requery/requery/wiki/Defining-Entities">requery wiki on model definitions</a>
+ */
+package org.mercury_im.messenger.data.model;

data/src/main/java/org/mercury_im/messenger/data/repository/package-info.java

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+/**
+ * The repository package contains implementations of the repositories defined in the domain
+ * module.
+ *
+ * While the data module uses requery to store data in an SQL database, the repositories use
+ * mappers defined in the mapping package to map the requery models to entities.
+ *
+ * Since the application itself only ever uses entities, it doesn't have to know about the database
+ * at all.
+ */
+package org.mercury_im.messenger.data.repository;