docs: Documentation draft for STM32F0

Signed-off-by: Eugene Krashtan <Eugene.Krashtan@opensynergy.com>

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+This document describes how the STM32F0 port operates and how to work with
+tiny CAN-enabled boards.
+
+Required components
+===================
+
+#### MCP2515 module
+
+![MCP2515](img/mcp2515.jpg)
+
+#### St-link dongle
+
+![st-link v2](img/stlinkv2-700x700.jpg)
+
+
+Adding CAN bus to Raspberry Pi
+==============================
+(Based on Quick Guide https://www.raspberrypi.org/forums/viewtopic.php?f=44&t=141052)
+
+First of all, it’s necessary to modify the CAN-module from ebay, because it has
+only one VCC pin, but the MCP2515 needs to be powered from 3V3 and the TJA1050
+CAN-transceiver needs to be powered from 5V. Powering both chips from 5V would
+work, but then a level-shifter for the SPI would be needed. The Pi's GPIO pins
+are NOT 5V tolerant. Cut a trace on the PCB and soldered a pin onto the trace
+to deliver 5V only to the TJA1050. Be sure to cut the trace before the capacitor:
+
+![VCC cut](img/mcp2515_vcc_cut.png)
+
+Next connect the module:
+
+| MCP2515 | Raspberry Pi |
+| --- | --- |
+| VCC | 1 (3V3) |
+| TJA 1050 VCC | 2 (5V) |
+| GND | 6 (GND) |
+| CS | 24 (CE0) |
+| MISO | 21 (MISO) |
+| MOSI | 19 (MOSI) |
+| SCK | 23 (SCK) |
+| INT | 22 (GPIO25) |
+
+Install can-utils:
+> sudo apt-get install can-utils
+
+To activate the driver for the MCP2515 you have to add a kernel overlay, to do
+so edit the /boot/config.txt
+> sudo nano /boot/config.txt
+
+And add the following lines (set oscillator value according to crystal on your board):
+
+```
+dtparam=spi=on
+dtoverlay=mcp2515-can0,oscillator=8000000,interrupt=25
+dtoverlay=spi1-1cs
+```
+
+Now reboot, after the reboot try to setup the the can interface:
+> sudo ip link set can0 up type can bitrate 500000
+
+If no errors occurred, the can interface should be ready now.
+To make the CAN-interface permanent, add the following lines to /etc/network/interfaces
+
+```
+auto can0
+iface can0 can static
+    bitrate 500000
+```
+
+Communicating over CAN
+======================
+
+Use "Serial over CAN" emulator software to establish connection:
+https://github.com/Delsian/CanSerial
+
+Install st-link on Raspberry Pi
+===============================
+
+> sudo apt-get update
+
+> sudo apt-get install cmake
+
+> sudo apt-get install libusb-1.0-0-dev
+
+> git clone https://github.com/texane/stlink stlink-repo
+
+> cd stlink-repo
+
+> make
+
+> cd build/Release/
+
+> sudo make install
+
+
+Copy to /etc/udev/rules.d/49-stlinkv2.rules:
+
+```
+# stm32 discovery boards, with onboard st/linkv2
+# ie, STM32L, STM32F4.
+# STM32VL has st/linkv1, which is quite different
+
+SUBSYSTEMS=="usb", ATTRS{idVendor}=="0483", ATTRS{idProduct}=="3748", \
+    MODE:="0666", \
+    SYMLINK+="stlinkv2_%n"
+
+SUBSYSTEMS=="usb", ATTRS{idVendor}=="0483", ATTRS{idProduct}=="374b", \
+    KERNEL!="sd*", KERNEL!="sg*", KERNEL!="tty*", SUBSYSTEM!="bsg", \
+    MODE:="0666", \
+    SYMLINK+="stlinkv2_%n"
+
+SUBSYSTEMS=="usb", ATTRS{idVendor}=="0483", ATTRS{idProduct}=="374b", \
+    KERNEL=="sd*", MODE:="0666", \
+    SYMLINK+="stlinkv2_disk"
+
+SUBSYSTEMS=="usb", ATTRS{idVendor}=="0483", ATTRS{idProduct}=="374b", \
+    KERNEL=="sg*", MODE:="0666", \
+    SYMLINK+="stlinkv2_raw_scsi"
+
+SUBSYSTEMS=="usb", ATTRS{idVendor}=="0483", ATTRS{idProduct}=="374b", \
+    SUBSYSTEM=="bsg", MODE:="0666", \
+    SYMLINK+="stlinkv2_block_scsi"
+
+SUBSYSTEMS=="usb", ATTRS{idVendor}=="0483", ATTRS{idProduct}=="374b", \
+    KERNEL=="tty*", MODE:="0666", \
+    SYMLINK+="stlinkv2_console"
+
+# If you share your linux system with other users, or just don't like the
+# idea of write permission for everybody, you can replace MODE:="0666" with
+# OWNER:="yourusername" to create the device owned by you, or with
+# GROUP:="somegroupname" and control access using standard unix groups.
+```
+
+Now "make flash" command can upload HEX into connected board
+
+Pins allocation
+===============
+
+Configurations with CAN and Serial port uses different pins, and firmware contains only
+actual pins initialization.