Make some updates based on first run

[jpolchlo]

We ran this workshop for some folks, and there are some things to check in on to improve the clarity. In no particular order.

1. Create a README for starting up.
2. Implicit discussion should precede type parameter discussion so that we can create some more examples.
3. Variance discussion should be beefed up. The variance block in type params notebook might want to be dropped.
4. The higher-kinded types might be well motivated by using Container to allow us to parameterize behavior (Container for Option allows keeping a single value or none at all, Lists with FIFO or LIFO behavior, Future for deferred/lazy compilation, etc)
5. Context bounds discussion may want to include <:
6. Type classes example in implicits section doesn't appear to use the Show trait?
7. Show how to use companion objects to limit scope
8. Include a section on map and flatmap?
9. Raster source notebook should pull geotrellis version 3.6.3 to avoid extra downloading