Update README

README.md

@@ -1,13 +1,24 @@
 # dapboot
 The dapboot project is an open-source USB [Device Firmware Upgrade](http://www.usb.org/developers/docs/devclass_docs/DFU_1.1.pdf) (DFU) bootloader for STM32 devices.
 
-Currently, the only target officially supported is the STM32F103x chip on STLink/v2 compatible boards.
+Currently, the only targets officially supported are the STM32F103x series.
 
 ## Build instructions
-As only one target is really in use, invoking `make` will build a binary for the STM32F103. The more general usage is:
+The default target is a generic STM32F103 dev board with an LED on PC13, commonly referred to as a "bluepill" board.
+
+To build other targets, you can override the
+`TARGET` variable when invoking `make`.
 
     make clean
-    make TARGET=STM32F103
+    make TARGET=STLINK
+
+### Targets
+
+| Target Name | Description | Link |
+| ----------- | ----------- |----- |
+|`BLUEPILL`   | Cheap dev board | http://wiki.stm32duino.com/index.php?title=Blue_Pill |
+|`MAPLEMINI`  | LeafLabs Maple Mini board and clone derivatives | https://github.com/rogerclarkmelbourne/Arduino_STM32/wiki/Maple-and-Maple-mini |
+|`STLINK`     | STLink/v2 hardware clones | https://wiki.paparazziuav.org/wiki/STLink#Clones |
 
 ## Flash instructions
 The `make flash` target will use openocd to upload the bootloader to an attached board. By default, the Makefile assumes you're using a [CMSIS-DAP](http://www.arm.com/products/processors/cortex-m/cortex-microcontroller-software-interface-standard.php) based probe, but you can override this by overriding `OOCD_INTERFACE` variable. For example:
@@ -17,9 +28,30 @@ The `make flash` target will use openocd to upload the bootloader to an attached
 ## Overriding defaults
 Local makefile settings can be set by creating a `local.mk`, which is automatically included.
 
-## Switching to the bootloader
+Here is an example `local.mk` that changes the default target to the STLink/v2 and uses an unmodified STLink/v2 to flash it.
+
+    TARGET ?= STLINK
+    OOCD_INTERFACE ?= interface/stlink-v2.cfg
+
+## Using the bootloader
+### Building for the bootloader
+The bootloader occupies the lower 8KiB of flash, so your application must offset its flash contents by 8KiB. This can be done by modifying your linker script or flags as appropriate.
+
+See the [highboot branch](https://github.com/devanlai/dapboot/tree/highboot) for an experimental variation that doesn't require the application to be offset.
+
+### Switching to the bootloader
 The bootloader can be built to look for arbitrary patterns, but the default for the STM32F103 target looks for a magic value stored in the RTC backup registers. Writing the magic value and then resetting will run the bootloader instead of the main application.
 
+The bootloader currently looks for `0x544F` in RTC backup register 1 and `0x4F42` in RTC backup register 0 (together they spell "BOOT" in ASCII).
+
+### WebUSB
+This bootloader implements the draft [WebUSB](https://wicg.github.io/webusb/) specification, which allows whitelisted origins to access the bootloader (with explicit user approval) on the following https origins:
+
+ * https://devanlai.github.io/
+ * https://localhost:8000/
+
+For a demo implementing dfu-util features in the browser, see https://devanlai.github.io/webdfu/dfu-util/
+
 ## USB VID/PID
 The USB VID/PID pair ([1209/DB42](http://pid.codes/1209/DB42/)) is allocated through the [pid.codes](http://pid.codes/) open-source USB PID program.