Add Buildpack Types Section to Buildpack Spec

[ekcasey]

The buildpack spec should clarify the different types of buildpacks (regular vs metabuidpack) and make clear when sections of the spec apply to specific types of buildpacks. This will make the spec more comprehensible for buildpack authors and help us norm on terminology.

As part of this issue we should pick clear unambiguous terminology for specific types of buildpacks and ensure we use that terminology consistently going forward.

[natalieparellano]

Please see #300 - it might be worth blocking this issue on that PR. Once we decide what terminology we like, we can propagate it everywhere.

[natalieparellano]

Closed in #300