Update readme: usage, features, system libnitrokey

This patch updates the readme with:
- more usage information, especially a pointer to the examples and to
  nitrocli,
- information about the supported features (= everything),
- information on how to use the system libnitrokey instead of building
  it from source.

1 files changed, 18 insertions, 7 deletions

diff --git a/README.md b/README.md
index 5f775ef..8ce093e 100644
--- a/README.md
+++ b/README.md
@@ -7,19 +7,28 @@ SPDX-License-Identifier: CC0-1.0
 
 A libnitrokey wrapper for Rust providing access to Nitrokey devices.
 
-[Documentation][]
+## Usage
+
+For usage information, have a look at the [API reference][API reference] and at
+the [examples][] in the `examples` directory.  You can also have a look at the
+[`nitrocli`][] crate, a command-line interface for Nitrokey devices that uses
+this crate.
 
 ## Compatibility
 
-The required [`libnitrokey`][] version is built from source.  The host system
+This crate provides access to all features of the [`libnitrokey`][] C API for
+both the Nitrokey Pro and the Nitrokey Storage: general configuration, one-time
+password generation, the password safe and the secure storage on the Nitrokey
+Storage.
+
+The required `libnitrokey` version is built from source.  The host system
 must provide `libhidapi-libusb0` (Linux) or `libhidapi` (non-Linux) in the
 default library search path.  Depending on your system, you might also have to
 install the [Nitrokey udev rules][].
 
-Currently, this crate provides access to the common features of the Nitrokey
-Pro and the Nitrokey Storage:  general configuration, OTP generation and the
-password safe.  Basic support for the secure storage on the Nitrokey Storage is
-available but still under development.
+If you want to use a precompiled version of `libnitrokey`, you can set the
+`USE_SYSTEM_LIBNITROKEY` environment variable during build.  In this case,
+`libnitrokey` must be available in the library search path.
 
 ### Unsupported Functions
 
@@ -92,7 +101,9 @@ in the `LICENSES` directory.  `libnitrokey` is licensed under the [LGPL-3.0][].
 
 `nitrokey-rs` complies with [version 3.0 of the REUSE specification][reuse].
 
-[Documentation]: https://docs.rs/nitrokey
+[API reference]: https://docs.rs/nitrokey
+[examples]: https://docs.rs/crate/nitrokey/0.4.0/source/examples/
+[`nitrocli`]: https://github.com/d-e-s-o/nitrocli/tree/master/nitrocli
 [Nitrokey udev rules]: https://www.nitrokey.com/documentation/frequently-asked-questions-faq#openpgp-card-not-available
 [`libnitrokey`]: https://github.com/nitrokey/libnitrokey
 [`nitrokey-test`]: https://github.com/d-e-s-o/nitrokey-test