Add high level crate documentation.

openpgp-card-sequoia/src/lib.rs

@@ -3,6 +3,45 @@
 
 //! A higher-level wrapper around the openpgp-card crate.
 //! It uses sequoia_openpgp for OpenPGP operations.
+//!
+//! # Backends
+//!
+//! To make use of this crate, you need to use a backend for communication
+//! with cards. The suggested default backend is `openpgp-card-pcsc`.
+//!
+//! With `openpgp-card-pcsc` you can either open all available cards:
+//!
+//! ```no_run
+//! use openpgp_card_pcsc::PcscClient;
+//! use openpgp_card_sequoia::card::Open;
+//!
+//! # fn main() -> Result<(), Box<dyn std::error::Error>> {
+//! for card in PcscClient::cards()? {
+//!     let open = Open::open_card(card)?;
+//!     println!("Found OpenPGP card with ident '{}'",
+//!              open.get_application_id()?.ident());
+//! }
+//! # Ok(())
+//! # }
+//! ```
+//!
+//! Or you can open one particular card, by ident:
+//!
+//! ```no_run
+//! use openpgp_card_pcsc::PcscClient;
+//! use openpgp_card_sequoia::card::Open;
+//!
+//! # fn main() -> Result<(), Box<dyn std::error::Error>> {
+//! let card = PcscClient::open_by_ident("abcd:12345678")?;
+//! let open = Open::open_card(card)?;
+//! # Ok(())
+//! # }
+//! ```
+//!
+//! # Use for cryptographic operations
+//!
+//! # Setting up and configuring a card
+//!
 
 use openpgp::packet::{key, Key};
 use sequoia_openpgp as openpgp;