docs: add an entry for LWT queries

The entry explains that lightweight transaction queries are
almost like regular queries, except they have a serial consistency level
in addition to the regular consistency level.

Author: psarna

docs/source/SUMMARY.md

@@ -19,6 +19,7 @@
     - [Prepared query](queries/prepared.md)
     - [Batch statement](queries/batch.md)
     - [Paged query](queries/paged.md)
+    - [Lightweight transaction query (LWT)](queries/lwt.md)
     - [USE keyspace](queries/usekeyspace.md)
     - [Schema agreement](queries/schema_agreement.md)
 

docs/source/queries/lwt.md

@@ -0,0 +1,32 @@
+# Lightweight transaction (LWT) query
+
+A lightweight transaction query can be expressed just like any other query, via `Session`, with the notable difference of having an additional consistency level parameter - the `serial_consistency_level`.
+
+
+### Format of the query
+A lightweight transaction query is not a separate type - it can be expressed just like any other queries: via `SimpleQuery`, `PreparedStatement`, batches, and so on. The difference lays in the query string itself - when it contains a condition (e.g. `IF NOT EXISTS`), it becomes a lightweight transaction. It's important to remember that CQL specification requires a separate, additional consistency level to be defined for LWT queries - `serial_consistency_level`. The serial consistency level can only be set to two values: `SerialConsistency::Serial` or `SerialConsistency::LocalSerial`. The "local" variant makes the transaction consistent only within the same datacenter. For convenience, Scylla Rust Driver sets the default consistency level to `LocalSerial`, as it's more commonly used. For cross-datacenter consistency, please remember to always override the default with `SerialConsistency::Serial`.
+```rust
+# extern crate scylla;
+# use scylla::Session;
+# use std::error::Error;
+# async fn check_only_compiles(session: &Session) -> Result<(), Box<dyn Error>> {
+use scylla::query::Query;
+use scylla::statement::{Consistency, SerialConsistency};
+
+// Create a Query manually to change the Consistency to ONE
+let mut my_query: Query = Query::new("INSERT INTO ks.tab (a) VALUES(?) IF NOT EXISTS".to_string());
+my_query.set_consistency(Consistency::One);
+// Use cross-datacenter serial consistency
+my_query.set_serial_consistency(Some(SerialConsistency::Serial));
+
+// Insert a value into the table
+let to_insert: i32 = 12345;
+session.query(my_query, (to_insert,)).await?;
+# Ok(())
+# }
+```
+
+The rest of the API remains identical for LWT and non-LWT queries.
+
+See [Query API documentation](https://docs.rs/scylla/0.2.0/scylla/statement/query/struct.Query.html) for more options
+