add new hardware peripheral

docs/hardware/blue-pill.md

@@ -13,12 +13,33 @@
 - `set_low()` usually turns it on
 - `set_high()` usually turns it off
 
-## Button Assumption
+## RGB LED
 
-- The repo button examples use `PA0`
-- Assumed wiring:
-  - button between `PA0` and `3V3`
-  - firmware enables internal pull-down
+- `PA5` = red
+- `PA6` = green
+- `PA7` = blue
+- `PA4` = drive low for the LED return path
+
+## 5-Way Button Board
+
+- `PB12` = GND, drive low
+- `PA10` = VCC, drive high
+- `PB13` = right
+- `PB14` = down
+- `PB15` = left
+- `PA8` = center
+- `PA9` = up
+- Assumption: the button inputs read high when pressed
+- The button examples use `PA8` as the default center button
+- TODO: flip the input polarity if your module is active-low instead
+
+## HMC5883L Module
+
+- `PB4` = VCC, drive high
+- `PB5` = GND, drive low
+- `PB6` = SCL
+- `PB7` = SDA
+- `PB8` = DRDY
 
 ## Probe Wiring