Improve clarity of the importance of certain GDK instructions

Merged username-removed-123742 requested to merge gsmethells/gitlab-development-kit:master into master

This change draws better attention to the necessary make command within the gitlab-development-kit by moving it up one markdown title level. I feel this is a necessary move because the previous section is essentially a list of platform choices, hence the reader will be intentionally skimming/skipping and could easily gloss over this important section.