docs: add pfSense installation guide

Author: miczyg1

docs/pfSense-install-guide.md

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+pfSense Installation Guide
+===========================
+
+## Contents
+
+<!-- TOC -->
+
+- [pfSense Installation Guide](#pfsense-installation-guide)
+    - [Contents](#contents)
+    - [pfSense Image](#pfsense-image)
+    - [Booting installer](#booting-installer)
+    - [Installation](#installation)
+
+<!-- /TOC -->
+
+
+## pfSense Image
+
+In order to install pfSense on apu2/3/5 platforms from USB, obtain following
+[Image](https://sgpfiles.pfsense.org/mirror/downloads/pfSense-CE-memstick-serial-2.4.2-RELEASE-amd64.img.gz)
+from official mirror.
+
+Extract the image somewhere and burn it onto usb stick using `dd`:
+
+```
+dd if=pfSense-CE-memstick-serial-2.4.2-RELEASE-amd64.img of/dev/sdx status=progress
+```
+
+where `sdx` is the correct USB drive.
+
+> On BIOS v4.6.4 or later and v4.0.14 or later one will have to deal with double
+> console redirection in newer BIOSes due to multiconsole option in installer.
+> In the [Installation section](#installation) there is a description how to fix
+> it. If one has a write access to the stick, just edit `/boot.config` to
+> `-S115200 -h` and in `/boot/loader.conf` change
+> `console="comconsole,vidconsole"` to `console="comconsole"` before booting
+> pfSense for the first time. Append `hint.ahci.0.msi="0"` to
+> `/boot/device.hints` file. Also it is good to change
+> `/boot/defaults/loader.conf`:
+>
+> - change `#boot_serial=""` to `boot_serial="YES"`
+> - change `#comconsole_speed="9600"` to `comconsole_speed="115200"`
+> - change `#console="vidconsole"` to `console=comconsole`
+>
+> Already prepared image is available at [3mdeb cloud](https://cloud.3mdeb.com/index.php/s/I36WY5x8pDcTd9y)
+
+## Booting installer
+
+Plug the USB stick prepared earlier to apu and boot from it.
+
+When main installer menu pops up do the following:
+
+- for BIOS v4.0.13 or earlier let it boot automatically
+- for BIOS v4.6.3 or earlier, interrupt the installer by pressing `ESC` and type
+  following commands:
+
+  ```
+  set hint.ahci.0.msi="0"
+  boot
+  ```
+
+> v4.6.x BIOSes need to have MSI disabled, due to signal races causing disk
+> write commands timeouts. This issue is not present in legacy BIOSes v4.0.x
+> Do it if did not edit `/boot/device.hints` earlier.
+
+Installer should load the kernel now and begin installation process.
+
+## Installation
+
+Proceed with the installation choosing the options that fit you.
+
+> Only for installation on BIOS v4.6.4 or later and v4.0.14 or later
+
+At the end of installation open the shell to customize the system:
+
+```
+echo "-S115200 -h" > /boot/config
+rm /boot.config
+ln /boot/config /boot.config
+```
+
+> It is necessary to create hardlink, because /boot.config seems to not be
+> persistent between boots.
+
+Copy `/boot/loadef.conf` and name it as `/boot/loadef.conf.local`.
+
+```
+cp /boot/loader.conf /boot/loader.conf.local
+```
+
+Open `/boot/loadef.conf.local`, change `console="comconsole,vidconsole"` to
+`console="comconsole"` and delete `boot_multicons="YES"` line. Append
+`hint.ahci.0.msi="0"` at the end and save.
+
+> This is needed for BIOS v4.6.4 or later and v4.0.14 or later to avoid doubled
+> output in loader. `hint.ahci.0.msi="0"` is needed when system hangs or reboots
+> after few hours uptime.
+
+Edit `/boot/defaults/loader.conf`:
+- change `#boot_serial=""` to `boot_serial="YES"`
+- change `#comconsole_speed="9600"` to `comconsole_speed="115200"`
+- change `#console="vidconsole"` to `console=comconsole`
+- change `loader_conf_files="/boot/device.hints /boot/loader.conf /boot/loader.conf.local"`
+  to `loader_conf_files="/boot/device.hints /boot/loader.conf.local"`
+