Add update info

[ci skip]

doc/gitlab-geo/README.md

@@ -62,6 +62,10 @@ If you installed GitLab from source:
 
 [install-ee-source]: https://docs.gitlab.com/ee/install/installation.html "GitLab Enterprise Edition installation from source"
 
+## Updating the Geo nodes
+
+Read how to [update your Geo nodes to the latest GitLab version](updating_the_geo_nodes.md).
+
 ## Current limitations
 
 - You cannot push code to secondary nodes

doc/gitlab-geo/updating_the_geo_nodes.md

+# Updating the Geo nodes
+
+In order to update the GitLab Geo nodes when a new GitLab version is released,
+all you need to do is update GitLab itself:
+
+1. Log into each node (primary and secondaries)
+1. Upgrade GitLab
+1. Test primary and secondary nodes, and check version in each.
+
+---
+
+For Omnibus GitLab installations it's a matter of updating the package:
+
+```
+# Debian/Ubuntu
+sudo apt-get update
+sudo apt-get install gitlab-ee
+
+# Centos/RHEL
+sudo yum install gitlab-ee
+```
+
+For installations from source, [follow the instructions for your GitLab version]
+(https://gitlab.com/gitlab-org/gitlab-ee/tree/master/doc/update).