Add docs and release note for CentOS 8

Adds info on supported distros to support matrix. Adds release note
about CentOS 7 drop and dropped images.

Change-Id: I0ada13af2fe57de550c1703d636436190e5d489a
Partially-Implements: blueprint centos-rhel-8

1 files changed, 27 insertions, 0 deletions

diff --git a/doc/source/support_matrix.rst b/doc/source/support_matrix.rst
index 090adc0e0..71ed26488 100644
--- a/doc/source/support_matrix.rst
+++ b/doc/source/support_matrix.rst
@@ -4,6 +4,33 @@
 Kolla Images Support Matrix
 ===========================
 
+This page describes the supported base container image distributions and
+versions, and the Kolla images supported on each of those.
+
+.. _support-matrix-base-images:
+
+Supported base images
+=====================
+
+The following base container images are supported:
+
+.. note::
+
+   CentOS 7 is no longer supported as a base container image. The Train release
+   supports both CentOS 7 and 8 images, and provides a route for migration.
+
+============= ============ ================
+Distribution  Default base Default base tag
+============= ============ ================
+CentOS 8      centos       8
+Debian Buster debian       10
+RHEL 8        rhel         8
+Ubuntu Bionic ubuntu       18.04
+============= ============ ================
+
+The remainder of this document outlines which images are supported on which of
+these distribution.
+
 Support clause definitions
 ==========================