Gpio cdev add documentation

Author: almusil

README.md

@@ -8,6 +8,17 @@ This project is developed and maintained by the [Embedded Linux team][team].
 
 ## [Documentation](https://docs.rs/linux-embedded-hal)
 
+## GPIO character device
+
+Since Linux kernel v4.4 the use of sysfs GPIO was deprecated and replaced by the character device GPIO.
+See [gpio-cdev documentation](https://github.com/rust-embedded/gpio-cdev#sysfs-gpio-vs-gpio-character-device) for details.
+
+This crate includes feature flag `gpio_cdev` that replaces [sysfs_gpio](https://crates.io/crates/sysfs_gpio) by [gpio-cdev](https://crates.io/crates/gpio-cdev).
+To enable it update your Cargo.toml. 
+```
+linux-embedded-hal = { version = "0.3", features = ["gpio_cdev"] }
+``` 
+
 ## License
 
 Licensed under either of

src/lib.rs

@@ -11,6 +11,7 @@
 //! [0]: https://crates.io/keywords/embedded-hal
 
 #![deny(missing_docs)]
+#![feature(doc_cfg)]
 
 extern crate cast;
 extern crate embedded_hal as hal;
@@ -99,9 +100,11 @@ impl hal::blocking::delay::DelayMs<u64> for Delay {
 ///
 /// [`gpio_cdev::LineHandle`]: https://docs.rs/gpio-cdev/0.2.0/gpio_cdev/struct.LineHandle.html
 #[cfg(feature = "gpio_cdev")]
+#[doc(cfg(feature = "gpio_cdev"))]
 pub struct Pin(pub gpio_cdev::LineHandle, bool);
 
 #[cfg(feature = "gpio_cdev")]
+#[doc(cfg(feature = "gpio_cdev"))]
 impl Pin {
     /// See [`gpio_cdev::Line::request`][0] for details.
     ///