examples: Add example of I2C usage, taking PyBoard accelerometer as subject.

examples/accel_i2c.py

+# This is an example on how to access accelerometer on
+# PyBoard directly using I2C bus. As such, it's more
+# intended to be an I2C example, rather than accelerometer
+# example. For the latter, using pyb.Accel class is
+# much easier.
+
+import pyb
+import time
+
+# Accelerometer needs to be powered on first. Even
+# though signal is called "AVDD", and there's separate
+# "DVDD", without AVDD, it won't event talk on I2C bus.
+accel_pwr = pyb.Pin("MMA_AVDD")
+accel_pwr.value(1)
+
+i2c = pyb.I2C(1)
+addrs = i2c.scan()
+print("Scanning devices:", [hex(x) for x in addrs])
+if 0x4c not in addrs:
+    print("Accelerometer is not detected")
+
+ACCEL_ADDR = 0x4c
+ACCEL_AXIS_X_REG = 0
+ACCEL_MODE_REG = 7
+
+# Now activate measurements
+i2c.mem_write(b"\x01", ACCEL_ADDR, ACCEL_MODE_REG)
+
+print("Try to move accelerometer and watch the values")
+while True:
+    val = i2c.mem_read(1, ACCEL_ADDR, ACCEL_AXIS_X_REG)
+    print(val[0])
+    time.sleep(1)