Add builders information to buildpacks page

[anthonydahanne]

a buildpack references one/several stacks and both are displayed in any buildpack page (such as https://registry.buildpacks.io/buildpacks/fagiani/apt), but what about someone who’d want to publish a buildpack relying on stacks that are not as popular as heroku or paketo or google stacks... 

if the page does not display which builder includes such  stacks, how can consumers find out which builder they need to use?

[jromero]

@jkutner / @elbandito: I know there was an ongoing discussion about this. Where are we with it? Is it something coming soon?

[jkutner]

Still on our minds, but we haven't moved it forward yet. I'm also curious to see how buildpacks/rfcs#167 lands, so I might let that shake out first since.