Document GPIO

You mean:
```rust
/// General Purpose I/O
pub trait GPIO {
  /// Set to logic high
  fn set_high(&self);

  /// Set to logic low
  fn set_low(&self);

  /// Read current logic level
  fn level(&self) -> GPIOLevel;

  /// Set directio to `Low` or `High`
  fn set_direction(&self, new_mode: GPIODirection);
}
```