Add release notes for changes since 3.0.0 release

We decided to use reno for coming releases, but we don't
have entries for changes made after the previous release
was shipped. This commit adds notes for them.

Also fixes the wrong place of "Release Notes" section in
the document index. It was my mistake in the past commit.

Change-Id: Ie0eedf8cb82a57db070a65f73d965f1a3ce378ab

releasenotes/notes/relnotes-from-3.0.0-d7306f5af5e3868d.yaml

@@ -0,0 +1,25 @@
+---
+features:
+  - Support os-client-config. OS_CLOUD environment variable is used for
+    selecting named cloud configuration.
+  - Support keystoneauth1 library which brings us better kyestone v3 support.
+  - Client command extension now supports a child resource.
+  - New CLI for VPNaaS multiple local subnets.
+  - New CLI for VPNaaS endpoint group API.
+  - New CLI for flavor argument to loadbalancer v2 create.
+  - New CLI for Neutron flavor framework.
+  - Support creating floating IP on a specific subnet ID.
+  - NSX gateway extension adds new transport type values (ipsec_gre and ipsec_stt).
+  - New router-update option to update static routes (--route and --no-routes).
+  - New allowed-address-pairs option to port-update
+upgrade:
+  - Cisco-specific neutron client commands have been removed.
+    These commands are ported to networking-cisco.
+  - py26 support has been dropped.
+fixes:
+  - Name is no longer looked up on RBAC policies,
+    RBAC policies have no name field so the name query to
+    the server was always returning all entries since the
+    name filter was ignored. (bug 1517818)
+other:
+  - cliff-tablib has been removed from test dependencies.