Add TLS to documentation book
# TLS
-TLS support is still in development, see pull request [#191](https://github.com/scylladb/scylla-rust-driver/pull/191)
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+Driver uses the [`openssl`](https://github.com/sfackler/rust-openssl) crate for TLS functionality.
+It was chosen because [`rustls`](https://github.com/ctz/rustls) doesn't support certificates for ip addresses
+(see [issue](https://github.com/briansmith/webpki/issues/54)), which is a common use case for Scylla.
+
+
+### Enabling feature
+`openssl` is not a pure Rust library so you need enable a feature and install the proper package.
+
+To enable the `tls` feature add in `Cargo.toml`:
+```toml
+scylla = { git = "https://github.com/...", branch = "main", features = ["ssl"] }
+openssl = "0.10.32"
+```
+
+Then install the package with `openssl`:
+* Debian/Ubuntu:
+ ```bash
+ apt install libssl-dev pkg-config
+ ```
+* Fedora:
+ ```bash
+ dnf install openssl-devel
+ ```
+<!--
+ scylla-rust-driver doesn't build on Alpine, some strange cc linker errors in proc-macro-hack 0_o
+ TODO: try building and add the section
+
+ * Alpine:
+ ```bash
+ apk add openssl-dev
+ ```
+-->
+* Arch:
+ ```bash
+ pacman -S openssl pkg-config
+ ```
+
+### Using TLS
+To use tls you will have to create an openssl
+[`SslContext`](https://docs.rs/openssl/0.10.33/openssl/ssl/struct.SslContext.html)
+and pass it to `SessionBuilder`
+
+For example, if database certificate is in the file `ca.crt`:
+```rust
+# extern crate scylla;
+# extern crate openssl;
+use scylla::{Session, SessionBuilder};
+use openssl::ssl::{SslContextBuilder, SslMethod, SslVerifyMode};
+use std::path::PathBuf;
+
+# use std::error::Error;
+# async fn check_only_compiles() -> Result<(), Box<dyn Error>> {
+let mut context_builder = SslContextBuilder::new(SslMethod::tls())?;
+context_builder.set_ca_file("ca.crt")?;
+context_builder.set_verify(SslVerifyMode::PEER);
+
+let session: Session = SessionBuilder::new()
+ .known_node("127.0.0.1:9142") // The the port is now 9142
+ .ssl_context(Some(context_builder.build()))
+ .build()
+ .await?;
+
+# Ok(())
+# }
+```
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