tips: add debug_hook.md with netcat example

tips/debug_hook.md

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+# Inspecting Initramfs
+
+See [PmOS Wiki](https://wiki.postmarketos.org/wiki/Inspecting_the_initramfs)
+
+
+## Enable debug hook
+
+```
+pmbootstrap initfs hook_add debug-shell
+pmbootstrap flasher flash_kernel # or however you flash/boot the kernel
+```
+
+
+## Connect to device using Telnet
+
+Running:
+
+```
+telnet 172.16.42.1
+```
+
+opens a shell to device initramfs.
+
+
+## Mount rootfs
+
+Current root filesystem is initramfs loaded in RAM.
+To switch to device root filesystem run:
+
+```
+source ./init_functions.sh
+mount -o rw $(find_root_partition) /sysroot
+``` 
+
+Now you have mounted rootfs inside `/sysroot` directory
+
+**NOTE**: to continue boot you have to undo this operation and let init system mount root partition by itself so run:
+
+```
+umount /sysroot
+pmos_continue_boot
+```
+
+
+## Chroot into rootfs
+
+Once you have mounted to `/sysroot` you can run programs in it.
+First do a chroot:
+
+```
+chroot /sysroot
+```
+
+
+Now you can run executable as if you booted.
+
+**NOTE**: services are not running and would not start correctly inside chroot
+
+To undo just run from chroot shell:
+
+```
+exit
+```
+
+
+## Copy files folders from device to PC (netcat)
+
+From device go into folder you want to send, then run:
+
+```
+tar c . | nc -l -p 10000
+```
+
+From PC go to destination folder and run:
+
+```
+nc -w 5 172.16.42.1 10000 | tar xf -
+```
+
+**NOTE**: `-w 5` otpions is a timeout of 5 seconds needed because sometimes `nc` does keep the connections after all data is sent so it would never quit.