adding update info regarding gitlab-shell config.yml (re: dashboard update problems)

Created by: TrevorS

Commit gitlabhq/gitlab-shell@93bfff7b3fc30fc362b90c4d362528f6e49786e6 from Chris adds remote redis support to gitlab-shell.

Because of the new feature, additional configuration options must be specified in gitlab-shell's config.yml.

The documentation that describes upgrading from GitLab 5.0 to 5.1 mentions updating gitlab-shell to v1.3.0 but does not inform the upgrader that changes have been made to config.yml and that the user's existing config.yml needs to be updated with the new options.

Without the redis configuration options, I encountered issues similar to: #4027, #3633, #4061, and #3807.

After updating gitlab-shell's config.yml with the new redis options, the dashboard updates correctly again.