Merge pull request #3 from 107-systems/doc

Minimal documentation how to use the features of this library.

Author: aentinger

README.md

@@ -11,14 +11,35 @@
 
 This library provides support functionality for building a complete [Cyphal](https://opencyphal.org/) application in combination with [107-Arduino-Cyphal](https://github.com/107-systems/107-Arduino-Cyphal).
 
-**Features:**
-* API for obtaining a unique 64-bit ID.
-* API for permanent register storage and retrieval.
-
 <p align="center">
   <a href="https://github.com/107-systems/l3xz"><img src="https://raw.githubusercontent.com/107-systems/.github/main/logo/l3xz-logo-memento-mori-github.png" width="30%"></a>
   <a href="https://github.com/107-systems/viper"><img src="https://github.com/107-systems/.github/raw/main/logo/viper.jpg" width="30%"></a>
 </p>
 
 This library works for
 * [arduino-pico](https://github.com/earlephilhower/arduino-pico): [`Raspberry Pi Pico`](https://www.raspberrypi.org/products/raspberry-pi-pico), `Adafruit Feather RP2040`, ... :heavy_check_mark:
+
+**Features:**
+* API for obtaining a **unique 64-bit ID**.
+```C++
+auto /* std::array<uint8_t, 16> */ const UNIQUE_ID = cyphal::support::UniqueId::instance().value();
+```
+* API for **persistent register storage and retrieval**.
+```C++
+/* Declaration of key/value storage. */
+cyphal::support::platform::storage::littlefs::KeyValueStorage kv_storage(filesystem);
+
+/* Load persistently stored registers from a non-volatile memory (EEPROM, flash, etc.). */
+if (auto const opt_err = cyphal::support::load(kv_storage, *node_registry); opt_err.has_value())
+{
+  Serial.print("load failed with error code ");
+  Serial.println(static_cast<int>(opt_err.value()));
+}
+
+/* Store persistent registers to a non-volatile memory (EEPROM, flash, etc.). */
+if (auto const opt_err = cyphal::support::save(kv_storage, *node_registry); opt_err.has_value())
+{
+  Serial.print("save failed with error code ");
+  Serial.println(static_cast<int>(opt_err.value()));
+}
+```