Book: add authentication documentation
# Authentication
-Authentication is still in development, see pull request [#177](https://github.com/scylladb/scylla-rust-driver/pull/177)
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+Driver supports authentication by username and password.
+
+To specify credentials use the `user` method in `SessionBuilder`:
+
+```rust
+# extern crate scylla;
+# extern crate tokio;
+# use std::error::Error;
+# async fn check_only_compiles() -> Result<(), Box<dyn Error>> {
+use scylla::{Session, SessionBuilder};
+
+let session: Session = SessionBuilder::new()
+ .known_node("127.0.0.1:9042")
+ .user("myusername", "mypassword")
+ .build()
+ .await?;
+
+# Ok(())
+# }
+```
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