Add docs.

Author: WaVEV

docs/topics/indexes.rst

@@ -0,0 +1,22 @@
+Indexes from Expressions
+========================
+
+Django MongoDB Backend now supports creating indexes from expressions.
+Currently, only ``F()`` expressions are supported, which allows referencing
+fields from the top-level model inside embedded fields.
+
+Example::
+
+    from django.db import models
+    from django.db.models import F
+
+    class Author(models.EmbeddedModel):
+        name = models.CharField()
+
+    class Book(models.Model):
+        author = models.EmbeddedField(Author)
+
+        class Meta:
+            indexes = [
+                models.Index(F("author__name")),
+            ]