Added an example for i2c (#69)

* Added an example for i2c

* added more extensive comments

examples/i2c_find_address.rs

@@ -0,0 +1,56 @@
+#![no_main]
+#![no_std]
+
+#[allow(unused)]
+use panic_halt;
+
+use stm32f0xx_hal as hal;
+
+use crate::hal::{
+    prelude::*,
+    i2c::I2c,
+    stm32,
+};
+
+use cortex_m_rt::entry;
+
+/* Example meant for stm32f030xc MCUs with i2c devices connected on PB7 and PB8 */
+
+#[entry]
+fn main() -> ! {
+    if let Some(p) = stm32::Peripherals::take() {
+        cortex_m::interrupt::free(move |cs| {
+            let mut flash = p.FLASH;
+            let mut rcc = p.RCC.configure().freeze(&mut flash);
+
+            let gpiob = p.GPIOB.split(&mut rcc);
+
+            // Configure pins for I2C
+            let sda = gpiob.pb7.into_alternate_af1(cs);
+            let scl = gpiob.pb8.into_alternate_af1(cs);
+
+            // Configure I2C with 100kHz rate
+            let mut i2c = I2c::i2c1(p.I2C1, (scl, sda), 100.khz(), &mut rcc);
+
+            let mut _devices = 0;
+            // I2C addresses are 7-bit wide, covering the 0-127 range
+            for add in 0..=127 {
+                // The write method sends the specified address and checks for acknowledgement;
+                // if no ack is given by the slave device the result is Err(), otherwise Ok()
+                // Since we only care for an acknowledgement the data sent can be empty
+                match i2c.write(add, &[]) {
+                    Ok(_) => {
+                        _devices += 1;
+                    },
+                    Err(_) => (),
+                }
+            }
+
+            // Here the variable "_devices" counts how many i2c addresses were a hit
+       });
+    }
+
+    loop {
+        continue;
+    }
+}