Update requirements.md to make 8GB the minimum instance size
omnibus-gitlab#3858 (closed) has a good analysis of a bare minimum GitLab install (2.7 GB). That doesn't include the runtime memory you need for when Ruby code actually runs,
gitoperations, Redis, and PostgreSQL. A
git cloneof the Linux repo can take 2 GB RAM, and people often hit out of memory errors just importing a big repo like that.