Update doc/ to include notes about the filesystem and loading multiple game-dirs in sequence

doc/filesystem

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+NUCLIDE FILESYSTEM DOCUMENTATION
+
+The 'nuclide' shell script is the launcher.
+It sets PATH to include the directory 'bin' which contains the engine that
+you've built with build_engine.sh.
+
+When nuclide is run and executes the engine, it'll first read default.fmf which
+is a manifest file the engine reads. It is updated occasionally.
+It defines which folders to mount in the virtual filesystem of the engine
+and has a document entirely dedicated to itself. For that please read:
+	src/engine/specs/fte_manifests.txt
+
+On its own, Nuclide launches the game 'base', unless you tell it otherwise:
+./nuclide -game mygame
+You can also load multiple additional folders in a specific order by specifying
+multiple '-game' arguments.
+
+Will load 'mygame' instead of base.
+It will still load the other 'BASEGAME' entries listed in the default.fmf.
+You can even load your own manifest file over default.fmf, by passing
+	./nuclide -manifest mymanifest.fmf
+
+Once the game has loaded, it'll load the persistent menu.dat into our QuakeC
+progs VM.
+It' always running, you can make your own by forking src/menu-fn, src/menu-vgui
+or write something from scratch.
+
+When a game is mounted, we're either looking for loose files (loaded last), or
+pak archives the engine supports. Plain .pak, or zip archives with the pk3 or
+pk4 extensions are supported.
+Folders with the .pk3dir extensions are treated as if they were .pk3 files.
+The editor also supports .pk3dir folders.
+
+Nuclide contains many custom definition files that are not engine specific.
+.efx, .font, .sndshd and .way to name a few.
+The engine doesn't really read them, the game-logic mostly handles them.