Add a skeleton for bsnes user documentation.

As I was doing the research for #51, I figured I should write down what I
learned before I forgot it, and if I was going to do that, I might as well
write it down in Markdown so we can make a website from it.

docs/README.md

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+# bsnes User Documentation
+
+bsnes is a multi-platform Super Nintendo (Super Famicom) emulator, originally
+developed by [byuu](https://byuu.org/about), which focuses on performance,
+features, and ease of use.

docs/how-to/manifest.md

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+Override a game's memory map
+============================
+
+In order to correctly emulate
+how a real Super Nintendo would play a particular game,
+bsnes [needs to know](https://byuu.net/cartridges/boards/)
+how that game's data would be arranged
+in the system's memory map,
+and any extra hardware that might have been present in the game's cartridge.
+This extra data describing a game
+is called a "manifest".
+When searching for manifest data,
+bsnes uses the first source it finds
+in the following list:
+
+ 1. The [SNES Preservation Project][snespres] database provided with bsnes
+      - This provides accurate data for most commercially released games
+ 1. A manifest file stored beside the ROM file
+      - Allows you to control the exact memory map the emulated game will see
+ 1. Heuristic analysis of the ROM file
+      - Works for all commercially released games,
+        but often broken for prototypes, betas, mods, and homebrew games
+
+[snespres]: https://byuu.org/preservation
+
+Where to get a manifest
+-----------------------
+
+The easiest place to get a working manifest
+is from the included game database.
+Open the [Database/Super Famicom.bml][gamedb] file,
+search for the name of a game that's similar
+to the one you want to get working,
+then copy the entire "game" block.
+
+[gamedb]: https://github.com/bsnes-emu/bsnes/blob/master/bsnes/Database/Super%20Famicom.bml
+
+You should wind up with something that looks like this:
+
+    game
+      sha256:   12b77c4bc9c1832cee8881244659065ee1d84c70c3d29e6eaf92e6798cc2ca72
+      label:    Super Metroid
+      name:     Super Metroid
+      region:   SNS-RI-USA
+      revision: SHVC-RI-0
+      board:    SHVC-BA3M-01
+        memory
+          type: ROM
+          size: 0x300000
+          content: Program
+        memory
+          type: RAM
+          size: 0x2000
+          content: Save
+
+Alternatively,
+if you don't know the name of a similar game,
+or if the same game has multiple different entries
+(for example, a USA and a Europe release),
+you can use the Manifest Viewer
+to copy the manifest from a working game.
+
+Load up the working game in bsnes,
+then from the Tools menu,
+choose "Manifest Viewer",
+and you'll see the complete manifest bsnes is using.
+Again, you can copy the "game" block
+(ignore the separate "board" block below it).
+
+How to modify it
+----------------
+
+You should paste the manifest
+into a "plain text" editor to modify it,
+like Notepad++ on Windows,
+or TextEdit (in plain-text mode) on macOS.
+Regular old Notepad may work if you're using a version
+that can save files with Unix (LF) line-endings.
+Don't use a word processor that can format text,
+like Microsoft Word or Wordpad,
+since bsnes doesn't understand formatting codes
+and it will get confused.
+
+You can remove the lines beginning with `sha256:`, `name:`, and `revision:`,
+if present,
+since they're only useful for tracking
+which games are included in the official database.
+The text after `label:` controls
+the name that bsnes will display in the title-bar
+when the game is loaded.
+The text after `region:` tells bsnes
+whether the game expects to be run on an NTSC system
+(Japan, USA)
+or a PAL system (Europe, Australia).
+In the game database,
+this field holds the publisher's unique identifier
+from which bsnes infers the correct region,
+but you can just write "NTSC" or "PAL".
+
+You should also check the "memory" block with `type: ROM`.
+The `size:` field describes the expected size
+of the ROM image, in bytes.
+In our example above,
+the size is 0x300000 in hexadecimal,
+which is 3,145,728 in decimal,
+which is the exact number of bytes in the Super Metroid ROM image.
+If the manifest you're using as an example
+mentions a *different* size
+than the ROM image you're trying to get working,
+it's probably the wrong example
+and you should choose a different one.
+
+Where to put it
+---------------
+
+Make sure you save your customised manifest
+as a plain text file,
+using Unix (LF) line endings,
+and the UTF-8 text encoding.
+
+If your ROM image is in a higan-style game folder
+(a directory with the `.sfc` extension,
+containing a `program.rom` file),
+save the manifest in that directory,
+named `manifest.bml`.
+
+If your ROM image is a single file
+(probably with the `.sfc` extension),
+save the manifest beside it with the `.bml` extension.
+For example,
+if the ROM you're trying to get working is called `myhomebrew.sfc`,
+save the manifest beside it as `myhomebrew.bml`.
+
+Troubleshooting
+---------------
+
+If your ROM now loads correctly, congratulations!
+
+If it's still broken,
+the easiest way to verify that bsnes is reading your manifest
+is to open the Manifest Viewer from the Tools menu.
+If you don't see the manifest data as you created it,
+make sure the manifest data is saved in the correct location,
+with the correct name,
+and in the correct file format.
+
+Also,
+note that bsnes ignores external manifests
+for games that appear in its game database.
+
+If your manifest is being read correctly,
+but the game still doesn't work,
+you should try basing your manifest off a different game.

mkdocs.yml

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+site_name: bsnes User Documentation
+repo_url: https://github.com/bsnes-emu/bsnes/
+theme: readthedocs
+markdown_extensions:
+    - smarty
+    - toc:
+        permalink: true
+    - tables
+
+nav:
+    - Introduction: README.md
+    #- Tutorial:
+    - How-To Guides:
+         - Override a memory map: how-to/manifest.md
+    #- Concepts:
+    #- Reference: