docs: add info about CqlDecimal

Author: muzarski

docs/source/data-types/data-types.md

@@ -26,7 +26,7 @@ Database types and their Rust equivalents:
 * `Time` <----> `value::CqlTime`, `chrono::NaiveTime`, `time::Time`
 * `Timestamp` <----> `value::CqlTimestamp`, `chrono::DateTime<Utc>`, `time::OffsetDateTime`
 * `Duration` <----> `value::CqlDuration`
-* `Decimal` <----> `bigdecimal::Decimal`
+* `Decimal` <----> `value::CqlDecimal`, `bigdecimal::Decimal`
 * `Varint` <----> `value::CqlVarint`, `num_bigint::BigInt` (v0.3 and v0.4)
 * `List` <----> `Vec<T>`
 * `Set` <----> `Vec<T>`

docs/source/data-types/decimal.md

@@ -1,5 +1,39 @@
 # Decimal
-`Decimal` is represented as [`bigdecimal::BigDecimal`](https://docs.rs/bigdecimal/0.2.0/bigdecimal/struct.BigDecimal.html)
+`Decimal` is represented as `value::CqlDecimal` or [`bigdecimal::BigDecimal`](https://docs.rs/bigdecimal/latest/bigdecimal/struct.BigDecimal.html)
+
+## value::CqlDecimal
+
+Without any feature flags, the user can interact with `decimal` type by making use of `value::CqlDecimal` which is a very simple wrapper representing the value as signed binary number in big-endian order with a 32-bit scale.
+
+```rust
+# extern crate scylla;
+# use scylla::Session;
+# use std::error::Error;
+# async fn check_only_compiles(session: &Session) -> Result<(), Box<dyn Error>> {
+use scylla::IntoTypedRows;
+use scylla::frame::value::CqlDecimal;
+use std::str::FromStr;
+
+// Insert a decimal (123.456) into the table
+let to_insert: CqlDecimal =
+        CqlDecimal::from_signed_be_bytes_and_exponent(vec![0x01, 0xE2, 0x40], 3);
+session
+    .query("INSERT INTO keyspace.table (a) VALUES(?)", (to_insert,))
+    .await?;
+
+// Read a decimal from the table
+if let Some(rows) = session.query("SELECT a FROM keyspace.table", &[]).await?.rows {
+    for row in rows.into_typed::<(CqlDecimal,)>() {
+        let (decimal_value,): (CqlDecimal,) = row?;
+    }
+}
+# Ok(())
+# }
+```
+
+## bigdecimal::BigDecimal
+
+To make use of `bigdecimal::Bigdecimal` type, user should enable `bigdecimal-04` crate feature.
 
 ```rust
 # extern crate scylla;