Document the implicit resolution chain for codecs

[calvinlfer]

For example:
Given the following Cassandra Row

final case class CassandraRow(a: Int, b: Long, c: OuterData)
final case class OuterData(x: String, y: String, z: InnerData)
final case class InnerData(i: Int)

We can see CassandraRow contains nested data which will be stored as a UDT value and that OuterData also contains InnerData which is also stored as a UDT value, so we have a UDT value in a Row and a UDT value within a UDT value.

How do the implicits resolve to build a CqlDecoder?

• For a: Int -> we have a CqlPrimitiveDecoder[Int] - this gets summoned in the derivation (before going through the implicit conversion from PrimitiveDecoder -> CqlDecoder)

• For b: Long -> we have a CqlPrimitiveDecoder[Long] - this gets summoned in the derivation (before going through the implicit conversion from PrimitiveDecoder -> CqlDecoder)

• For c: OuterData -> this is more interesting, this ends up having a CqlPrimitiveDecoder[OuterData], but how does that get materialized?
  We get that through CqlUDTValueDecoder[OuterData]

If we step through the derivation of CqlUdtValueDecoder[OuterData], we'll find that it uses CqlPrimitiveDecoder to summon instances of String (x and y) and then you'll notice InnerData is also a UDTValue, so what ends up happening is you use CqlPrimitiveDecoder[InnerData] which ends up calling out to CqlUDTValueDecoder[InnerData] which ends up calling back out to CqlPrimitiveDecoder[Int] (the i: Int in InnerData).

You can see that CqlPrimitiveDecoder is a bridge that links all the mechanisms together
Perhaps this linking mechanism is unnecessary but this is how it currently works.