Add MongoDB example for linking documents.

Closes #631

mongodb/linking/README.md

@@ -0,0 +1,62 @@
+# Spring Data MongoDB - Linking Example
+
+This project contains examples for linking Documents using:
+
+* `@DBRef`
+* `@DocumentReference`
+
+## DBRef
+
+Use MongoDB native `dbref` for storing associations.
+
+```java
+class Manager {
+	
+    @DBRef
+    private List<Employee> employees;
+
+    // ...
+}
+```
+
+## DocumentReference
+
+Link documents via their id.
+
+```java
+class Manager {
+	
+    @DocumentReference
+    private List<Employee> employees;
+
+    // ...
+}
+```
+
+Link documents via a (non _id_) property.
+
+```java
+class Manager {
+	
+    @DocumentReference(lookup = "{ 'name' : ?#{#target} }")
+    private List<Employee> employees;
+
+    // ...
+}
+```
+
+Link documents JPA style.
+
+```java
+class Manager {
+
+    @ReadOnlyProperty
+    @DocumentReference(lookup="{'managerId':?#{#self._id} }")
+    private List<Employee> employees;
+
+    // ...
+}
+```
+----
+
+For more information please refer to the [Reference Documentation](https://docs.spring.io/spring-data/mongodb/docs/current/reference/html/#mapping-usage-references).