added ttl index docu (#298)

Author: hkernbach

docs/Drivers/Java/Reference/Collection/Indexes.md

@@ -203,6 +203,49 @@ IndexEntity index = collection.ensurePersistentIndex(Arrays.asList("a", "b.c"));
 // the index has been created with the handle `index.getId()`
 ```
 
+## ArangoCollection.ensureTtlIndex
+
+`ArangoCollection.ensureTtlIndex(Iterable<String> fields, TtlIndexOptions options) : IndexEntity`
+
+Creates a ttl index for the collection if it does not already exist.
+
+**Arguments**
+
+- **fields**: `Iterable<String>`
+
+  A list of attribute paths
+
+- **options**: `PersistentIndexOptions`
+
+  - **name**: `String`
+
+    Specify a custom name during index creation (optional). When running AQL queries you might then refer
+    to these name as your preferred index for lookups (e.g. Index Hints).
+
+  - **expireAfter**: `Integer`
+
+    The time (in seconds) after a document's creation after which the documents count as "expired".
+
+  - **inBackground**: `Boolean`
+
+    Indexes created with the 'inBackground' option, will not hold an exclusive collection
+    lock for the entire index creation period (rocksdb only).
+
+**Examples**
+
+```Java
+ArangoDB arango = new ArangoDB.Builder().build();
+ArangoDatabase db = arango.db("myDB");
+ArangoCollection collection = db.collection("some-collection");
+
+final TtlIndexOptions options = new TtlIndexOptions();
+		options.name("myTtlIndex");
+		options.expireAfter(3600);
+
+IndexEntity index = collection.ensureTtlIndex(Arrays.asList("a", "b.c"), options);
+// the index has been created with the handle `index.getId()`
+```
+
 ## ArangoCollection.getIndex
 
 `ArangoCollection.getIndex(String id) : IndexEntity`