explain in README doc that hvc models use hand-engineered features

[NickleDave]

so it's clear and there's no "gotcha" / "ah-ha" moment after staring at what the package does for 5 minutes.
A plot with features on it would be nice -- like the knn figure from the tutorial?

• add a pointer to vak saying "if you want algorithms that don't need hand-engineered features..."