Create contributor documentation explaining terminology/naming conventions

[marcelamelara]

From @joshuagl in #283 (review):

I'm worried we're not consistent in using capitalised names for components in the attestation model (Bundle, Attestation, Envelope, Statement, Predicate, etc). Should we do a consistency pass and create some contributor documentation explaining conventions like this?

[adityasaky]

I support this, I think it's also worth standardizing the corresponding Type URIs.

[tannerjones4075]

@marcelamelara Is this issue still relevant? I would like to contribute.

[marcelamelara]

@tannerjones4075 Yes, we'd totally welcome your contribution here, thank you!!