Docs: update Geo with upgrade instructions
See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html
What does this MR do?
Add info on upgrading Geo, for @stanhu - gitlab-com/www-gitlab-com!6239 (comment 35442459), https://gitlab.slack.com/archives/C3TRESYPJ/p1500597988745154
Stan pls check if the location makes sense and if it looks good to you? Pls feel free to merge directly if ready.