Added legacy loader documentation and planning

docs/legacy_loader.md

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+# Legacy Loader
+The legacy loader only is another code path which, in the end, merges together
+with the UEFI - code path again. It has no special purpose except of that one.
+
+
+
+## Order of Working
+The Nightloader's Stage 0 - Loader searchs for its partition which has the
+type `0x9d` in the MBR (Master Boot Record).
+
+It initializes a GDT (Global Descriptor Table) for going into 32-bit - mode,
+making it possible to use more of the RAM. This is particularily useful for
+loading the whole partition of the Stage 2 into RAM without having problems
+with the memory limitations of the 16-bit real mode (which has 20-bit addresses).
+
+Following that, it loads the 256MB of the NightLoader-partition into memory
+at address 0x8000, or 32768 in decimal. If the partition has another size than
+256MB, the Nightloader - Stage 0 will fail to load it.
+If the partition isn't formatted correctly, the boot process will fail.
+
+The Stage 0 - loader tries to find the path `/night/starter.bin` in its
+partition and starts that executable code, which it loads to address 0x8000,
+at its first byte.
+
+Now, the Starter is working. Aforementioned executable code has one purpose:
+load the ELF-executable of the actual Stage 2 - loader and start a function
+inside of it which is named `main`. If such function does not exist, it should
+display a debug-message which basically tells that the user needs a *very*
+skilled technician right about *yesterday*.
+
+
+
+## Current State
+The Nightloader:
+- can find its own partition in the Master Boot Record
+- writes an error message onto the screen if it couldn't find its own partition
+- does not go into 32-bit mode
+- only loads 8 sectors (4096 bytes) of the Stage 2 - partition into the RAM
+