Create tutorial

[chadoh]

Walk learners through three steps:

1. create basic smart contract using only the basic loam-subcontract-core
2. create more complex smart contract that make use of multiple subcontracts
3. build own subcontract

Might be an opportunity to use loam-subcontract-ft in there somewhere. Maybe as step 2, but walking people through implementing its whole interface might be too in-the-weeds for a high-level, end-to-end tutorial?

This may also entail creating a new subcontract, which could either just be for example/learning use or could actually be a useful subcontract (see #63).