Unlock max_instances_per_host in controller config

PROD-35769

Change-Id: Ifd639dca8a59964ec5a3d5f9f12b63736c594505
diff --git a/README.rst b/README.rst
index 0db3104..7b52e62 100644
--- a/README.rst
+++ b/README.rst
@@ -1552,3 +1552,15 @@
          all: True
          verbose: True
          all_cells: True
+
+
+Configure filter schedulers
+---------------------------
+At this moment only max_instances_per_host for NumInstancesFilter is supported.
+
+.. code-block:: yaml
+
+    nova:
+      controller:
+        filter_scheduler:
+          max_instances_per_host: 50