prettify readme

README → README.md

@@ -1,3 +1,6 @@
+Luftboot
+========
+
 This software is part of the Paparazzi UAV Project.
 
 Luftboot is the bootloader for stm32 based autopilots to enable upgrading the
@@ -6,11 +9,21 @@ autopilot firmware using usb. It implements the dfu standard.
 Luftboot is based on the usbdfu bootloader implementation by Gareth McMullin
 for the Black Magic Probe project (http://www.blacksphere.co.nz/main/blackmagic).
 
+Buildng
+-------
+
+Luftboot needs libopencm3 and an appropriate arm-none-eabi gcc cross compiler.
+If you don't have these in your PATH, specify them with e.g.
+    make LIBOPENCM3=path/to/libopencm3 PREFIX=path/to/bin/arm-none-eabi
+
 By default, Luftboot uses a 12MHz external clock to PLL it to 72MHz. If no external
 clock is available, or for any other reason, one can build Luftboot to use the 8MHz
 internal RC oscillator to PLL it to 48MHz. Call make as follows from inside ./src:
     make LUFTBOOT_USE_48MHZ_INTERNAL_OSC=1
 
+Startup sequence
+----------------
+
 At bootup, Luftboot uses several criteria (in this order) to decide whether to jump
 to the payload or to initiate the bootloader:
  * On boot, if the payload is NOT valid, start the bootloader
@@ -19,4 +32,4 @@ to the payload or to initiate the bootloader:
    start the bootloader
  * If the ADC2 pin on Lisa/M is grounded, jump to payload (i.e. "skip bootloader" jumper)
  * If voltage is present on the USB vbus, start the bootloader
- * Otherwise, jump to payload
+ * Otherwise, jump to payload