update readme with build instructions

readme.md

@@ -12,4 +12,21 @@ number in hexfile is uart baud rate
 - blinky_nrf52_rtscts_1M.hex blinky with uart terminal with flow control 
 
 # connect with minicom
-- minicom -D /dev/ttyAMA2 -b 1000000 -s
+- minicom -D /dev/ttyAMA2 -b 1000000 -s
+
+# build openocd on cm4:
+sudo apt update
+sudo apt upgrade -y
+sudo apt install git build-essential libtool autoconf texinfo libusb-1.0-0-dev libftdi1-dev libhidapi-dev pkg-config -y
+sudo apt-get install pkg-config libjim-dev -y
+
+git clone https://github.com/openocd-org/openocd.git
+cd openocd
+
+./bootstrap
+./configure --enable-bcm2835gpio --enable-sysfsgpio
+make
+sudo make install
+
+# test read/write nrf54l15 - does not work yet
+sudo openocd -f ./raspberrypi-swd0.cfg -f target/nordic/nrf54l.cfg -c "init" -c "reset halt" -c "mdb 0x00000000 2" -c "shutdown"