Add digital ocean guide link

Added [digital ocean guide][meta_guide] link, so folks will know where to ask for improvements/problems regarding installation steps, instead of creating new posts on meta.

[Inspiration for this PR came from [this post][meta_post], this should not have been a new post, instead it should be a comment on [digital ocean guide][meta_guide] post.]

[meta_guide]: https://meta.discourse.org/t/beginners-guide-to-deploy-discourse-on-digital-ocean-using-docker/12156
[meta_post]: https://meta.discourse.org/t/rack-mini-profiler-showing-up-in-production-for-admins/14735