Add upgrade notes for Geo in GitLab 10.0

[ci skip]

doc/gitlab-geo/README.md

@@ -71,6 +71,19 @@ internally by the secondary node to record what data has been replicated.
 
 In the secondary nodes there is an additional daemon: Geo Log Cursor.
 
+## Geo Requirements
+
+We highly recommend that you install Geo on an operating system that supports
+OpenSSH 6.9 or higher. The following operating systems are known to ship with a
+current version of OpenSSH:
+
+    * CentOS 7.4
+    * Ubuntu 16.04
+
+Note that CentOS 6 and 7.0 ship with an old version of OpenSSH that do not
+support a feature that Geo requires. See the [documentation on GitLab Geo SSH
+access](ssh.md) for more details.
+
 ### LDAP
 
 We recommend that if you use LDAP on your primary that you also set up a

doc/gitlab-geo/configuration.md

@@ -300,12 +300,25 @@ not create them manually.
 
 ### Upgrading Geo
 
-To avoid having to maintain consistency of the `authorized_keys` file for SSH access,
-we encourage all **Geo** users to
-[switch to SSH key lookups via the database](ssh.md).
-This will be necessary once Geo system hooks are removed.
+#### Upgrading to GitLab 10.0
+
+Since GitLab 10.0, we require all **Geo** systems to [use SSH key lookups via
+the database](ssh.md) to avoid having to maintain consistency of the
+`authorized_keys` file for SSH access. Failing to do this will prevent users
+from being able to clone via SSH.
+
+#### Upgrading from GitLab 9.3 or older
+
+If you started running Geo on GitLab 9.3 or older, we recommend that you
+resync your secondary PostgreSQL databases to use replication slots. If you
+started using Geo with GitLab 9.4 or 10.x, no further action should be
+required because replication slots are used by default. However, if you
+started with GitLab 9.3 and upgraded later, you should still follow the
+instructions below.
+
+When in doubt, it does not hurt to do a resync. The easiest way to do this in
+Omnibus is the following:
 
-We highly recommend using replication slots. The easiest way to do this in Omnibus is the following:
   1. Install GitLab on the primary server
   1. Run `gitlab-ctl reconfigure` and `gitlab-ctl restart postgresql`. This will enable replication slots on the primary database.
   1. Install GitLab on the secondary server.