Add documentation

[AlCutter]

No description provided.

[jyasskin]

When deciding to use this in another system, it'd be nice to see a description of the size of a proof, and the security properties of a proof that's usable offline. I assume the computational cost of verifying a proof is negligible?

[Martin2112]

The original question is about proof properties, that are possibly covered in the Verifiable Data Structures paper in the doc directory? We need to write additional documentation of various types.

[Martin2112]

Going to close this one with a reference to https://github.com/google/trillian/blob/master/docs/VerifiableDataStructures.pdf

I opened #491 to cover more documentation on installing and running a log.

[jyasskin]

The size of the proofs (ideally, in bytes rather than O()) and their online/offline requirements aren't in VerifiableDataStructures.pdf. I've forgotten what exactly I needed this for, so 🤷, but it's likely to be important for other users.