message: Update README.md: add section about Cassandra to Rust type mappings
diff: @@ -252,3 +252,21 @@ struct N {
  ```
  Look into this [link](https://github.com/AlexPikalov/into-cdrs-value-derive/tree/master/example) to find a full example how to use CDRS + [_into-cdrs-value-derive_](https://github.com/AlexPikalov/into-cdrs-value-derive) crate.
  +
  +### Mapping results into Rust structures
  +
  +In ordert to query information from Cassandra DB and transform results to Rust types an structures each row in a query result should be transformed leveraging one of following traits provided by CDRS `cdrs::types::{AsRustType, AsRust, IntoRustByName, ByName, IntoRustByIndex, ByIndex}`.
  +
  +* `AsRustType` may be used in order to transform such complex structures as Cassandra lists, sets, tuples. The Cassandra value in this case could non-set and null values.
  +
  +* `AsRust` trait may be used for similar purposes as `AsRustType` but it assumes that Cassandra value is neither non-set nor null value. Otherwise it panics.
  +
  +* `IntoRustByName` trait may be used to access a value as a Rust structure/type by name. Such as in case of rows where each column has its own name, and maps. These values may be as well non-set and null.
  +
  +* `ByName` trait is the same as `IntoRustByName` but value should be neither non-set nor null. Otherwise it panics.
  +
  +* `IntoRustByIndex` is the same as `IntoRustByName` but values could be accessed via column index basing on their order provided in query. These values may be as well non-set and null.
  +
  +* `ByIndex` is the same as `IntoRustByIndex` but value can be neither non-set nor null. Otherwise it panics.
  +
  +Relations between Cassandra and Rust types are described in [type-mapping.md](https://github.com/AlexPikalov/cdrs/blob/master/type-mapping.md). For details see examples.
  