Add some documentation about what's in various bsnes directories.

bsnes/README.md

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+bsnes source code
+=================
+
+A guide to what all these directories are for:
+
+  - [Database](./Database/)
+    contains the databases bsnes uses
+    to figure out what circuit board a game expects,
+    and also the database of pre-made game cheats
+  - [emulator](./emulator/)
+    contains the interface
+    that the emulation core in [sfc](./sfc/) implements
+      - It comes from higan v106,
+        which has many emulation cores
+        that all implement the same interface —
+        bsnes only has one,
+        but the interface is still a good abstraction,
+        so it's still here.
+  - [filter](./filter/)
+    contains classic CPU-based image upscaling filters,
+    like HQ2x and Super Eagle
+  - [gb](./gb/)
+    contains the SameBoy emulator code
+    used for Super Game Boy emulation
+  - [heuristics](./heuristics/)
+    contains the logic that guesses
+    which memory map a particular game expects
+    and what extra hardware it assumes is present,
+    when a game cannot be found in the database
+  - [Locale](./Locale/)
+    contains translation databases
+    that bsnes can use to display its user interface
+    in a different language
+  - [lzma](./lzma/)
+    contains the 7-Zip SDK
+    allowing bsnes to understad `.7z` archives
+  - [processor](./processor/)
+    contains all the CPU emulation cores
+    used by the hardware bsnes emulates
+      - Another holdover from higan v106,
+        where different supported systems
+        happen to use the same model CPU,
+        so the CPU emulation cores
+        are not specific to a system
+  - [sfc](./sfc/)
+    contains Super Famicom (SNES) emulation code
+  - [target-bsnes](./target-bsnes/)
+    contains the normal bsnes user interface
+  - [target-libretro](./target-libretro/)
+    implements the "libretro" API
+    on top of bsnes' native
+    [emulator](./emulator/) API,
+    so bsnes can be used with libretro-based frontends

bsnes/target-bsnes/README.md

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+bsnes user interface
+====================
+
+This directory contains the code for
+the normal bsnes user interface.
+
+  - [bsnes.cpp](./bsnes.cpp)
+    contains the `nall::main()` entry point
+  - [input](./input/)
+    contains the glue logic
+    that maps incoming controller inputs
+    into emulated controller inputs and hotkey signals
+  - [presentation](./presentation/)
+    contains the code that sets up the main window
+    (menu bar, status bar, etc.)
+  - [program](./program/)
+    contains the glue logic
+    that connects the UI to the emulation core,
+    generating manifests,
+    loading games,
+    displaying video and playing audio, etc.
+  - [resource](./resource/)
+    contains all the data bsnes uses at runtime,
+    like icons,
+    the about box logos,
+    the SNES board database
+  - [settings](./settings/)
+    contains Settings window stuff
+  - [tools](./tools/)
+    contains Tools window stuff
+