docs: added docs for vector type

docs/source/data-types/vector.md

@@ -0,0 +1,26 @@
+## Vector (for Cassandra only!)
+`Vector` is represented as `Vec<T>`
+
+```rust
+# extern crate scylla;
+# extern crate futures;
+# use scylla::Session;
+# use std::error::Error;
+# async fn check_only_compiles(session: &Session) -> Result<(), Box<dyn Error>> {
+use futures::TryStreamExt;
+
+// Insert a vector of ints into the table
+let my_vector: Vec<i32> = vec![1, 2, 3, 4, 5];
+session
+    .query_unpaged("INSERT INTO keyspace.table (a) VALUES(?)", (&my_vector,))
+    .await?;
+
+// Read a list of ints from the table
+let mut stream = session.query_iter("SELECT a FROM keyspace.table", &[])
+    .await?
+    .rows_stream::<(Vec<i32>,)>()?;
+while let Some((vector_value,)) = stream.try_next().await? {
+    println!("{:?}", vector);
+}
+# Ok(())
+# }