Add folder structure documentation for i386

As the project grows, it's a good idea to have an overview of the
project as a big picture. The newly added file will provide that big
picture for when the project is too big to read through for making a
small change. It contains unimplemented things in the described
folder structure (AHCI) as well as a documentation folder, which also
doesn't exist yet.

i386/folder-structure.md

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+# i386 Nightloader | Folder Structure
+
+## Legacy BIOS bootsector (bootsector/)
+
+Contains one file, the source of the bootsector. This assembly program
+must span the first 512 byte of a legacy Nightloader-booted Partition.
+
+## Legacy BIOS Edition (legacy-boot/)
+
+This edition of the Nightloader is made for older devices that predate
+UEFI (or potentially for devices that have unreliable UEFI-support).
+
+This folder contains the assembly sources to create the loader
+partition which contains raw machine code that is loaded by the
+bootsector and to which the control is handed thereafter.
+
+### Source Folder Structure
+
+#### drivers/
+
+- acpi  
+  Driver for the Avanced Control and Power Interface.
+- pci  
+  Driver for the Peripheral Component Interconnect. This is used to
+  first find and then interact with the builtin devices in a computer,
+  mostly storage devices, but also network cards, USB keyboards, etc..
+- ahci  
+  Driver for AHCI, the ATA Host Controller Interface (ATA meaning 
+  (IBM PC) AT Attachment), storage interface. This is used to interact
+  with practically every modern consumer SSD and HDD. Older HDDs might
+  use another interface, but that's not of elevated relevance.
+
+
+#### memory/
+
+Memory Management functions, A20 line check, GDT setup and, some day
+in the future, paging for add-ons execution. This also contains utils
+like the allocators (arena, pool) used throughout the bootloader.
+
+#### utility/
+
+Helper functions that make it easier to write the other functionality
+but don't fall into any of the other categories on their own.
+
+## Documentation (docs/)