Add charts instructions to developer documentation on gitlab.yml

https://docs.gitlab.com/omnibus/settings/gitlab.yml.html#adding-a-new-setting-to-gitlabyml has guidelines on Omnibus support when adding a new feature to gitlab.yml.

This can be misleading, though, if someone finds that and thinks they're 'done' with their new setting, as we should also add this to the Helm charts. (Having that documentation in Omnibus would be a bit weird, but it does need to be somewhere I guess.)

I think a good first step would be to point to the minimal places we need to update in the charts for this. It's more context-dependent, but on Slack https://gitlab.com/gitlab-org/charts/gitlab/blob/6f9b5dbcf62812a21d5a2b5bb5115ae076192e24/charts/gitlab/templates/_gitlab.yaml.tpl was given as a good starting point. We still need to actually use those in the templates for the containers, though, and also document the setting.