readme: Add basic instructions

Signed-off-by:  Eric Callahan <arksine.code@gmail.com>

README.md

@@ -1,2 +1,68 @@
 # CanBoot
- Can Bootloader for Klipper MCUs
+ Can Bootloader for STM32F103 MCUs
+
+ This bootloader is designed for CAN nodes to be used with
+ [Klipper](https://github.com/KevinOConnor/klipper).  The bootloader
+ itself makes use of Klipper's hardware abstraction layer, stripped
+ down to keep the footprint minimal.  Currently the bootloader
+ reserves 8 KiB of space, however it occupies less than 4 KiB.
+
+
+## Building
+
+CanBoot also uses Klipper's build system.  The build is configured
+with menuconfig.  The steps to fetch and build are as follows:
+```
+git clone https://github.com/Arksine/CanBoot
+cd CanBoot
+make menuconfig
+make
+```
+
+The menuconfig will present the following options:
+- `Processor model`: Currently only STM32F203 is supported
+- `Clock Reference`: Choose the appropriate setting for your board's crystal
+- `Flash Page Size`: The STM32F103 comes in multiple variants.  The low
+  density models have a 1 KiB page size, others a 2KiB page size.  Choose
+  the appropriate selection for your model
+- `CAN bus speed`: Select the appropriate speed for your setup
+- `Enable Status Led`: If enabled, the option to set the Status LED Pin is
+  made available
+- `Status LED GPIO Pin`:  The pin name for your LED.  The Pin can be inverted
+  if the LED is on when the pin is low.  For example, the status LED Pin for a
+  "blue pill" is !PC13.
+
+Once configured and built flash with a programmer such as an ST-Link
+
+## Uploading a Program
+
+1) Make sure [CanSerial](https://github.com/bondus/CanSerial) is installed and
+   enabled. Alternatively one can use the `pycanserial.py` script in this repo,
+   however it requries Python 3.7+.  It also lacks the "reset" feature of
+   CanSerial, so for the moment I recommend using C version linked.
+2) Enter the bootloader.  This may be accomplished through "double tapping" the
+   Reset button.  The second reset should be within 1.5 seconds of the first.
+   If the Status LED is set it will blink every second when the device has
+   entered the bootloader.  The bootloader should connect to CanSerial, and
+   CanSerial should create a PTY for it in the `/tmp` directory.
+3) Build Klipper with CAN support and with the "8KiB" bootloader setting enabled.
+4) Run the flash script:
+   ```
+   cd ~/CanBoot
+   ~/klippy-env/bin/python flash_can.py /tmp/ttyCANxxxxxxxx ~/klipper/out/klipper.bin
+   ```
+   Replace `/tmp/CANxxxxxxxx` with the appropriate symlink for your device.
+
+## Notes
+- This is preview code and will almost certainly change.  Specifically, it is
+  anticipated that the flash_can script will not use CanSerial in the future,
+  but instead connect directly to the CAN socket and issue commands to the
+  bootloader.  Likewise it is expected that it will be possible to enter the
+  booltoader via command from Klipper.  The IDs used to communicate with the
+  bootloader may also change (0x321, 0x322, 0x323).
+- If using a MCP2515 Can Device it is likely that packets will be dropped when
+  reading flash back from the node during the verification process.  Reducing
+  SPI speed may help with this, in my testing the process could not complete
+  at 10MHz.  If possible use a USB can device flashed with
+  [candlelight](https://github.com/candle-usb/candleLight_fw).
+