Add an "Incompatibilities between versions" page

This is largely based on our existing wiki page. I've pruned out a
couple of entries which I think are largely irrelevant (config file
splitting, Java API changes).

10/10 job by me of changing nothing since 1.13. Shame to break that
streak really.

doc/reference/breaking_changes.md

@@ -0,0 +1,67 @@
+---
+module: [kind=reference] breaking_changes
+---
+
+<!--
+SPDX-FileCopyrightText: 2019 The CC: Tweaked Developers
+
+SPDX-License-Identifier: MPL-2.0
+-->
+
+# Incompatibilities between versions
+
+CC: Tweaked tries to remain as compatible between versions as possible, meaning most programs written for older version
+of the mod should run fine on later versions.
+
+> [External peripherals][!WARNING]
+>
+> While CC: Tweaked is relatively stable across versions, this may not be true for other mods which add their own
+> peripherals. Older programs which interact with external blocks may not work on newer versions of the game.
+
+However, some changes to the underlying game, or CC: Tweaked's own internals may break some programs. This page serves
+as documentation for breaking changes and "gotchas" one should look out for between versions.
+
+## Minecraft 1.13 {#mc-1.13}
+ - The "key code" for [`key`] and [`key_up`] events has changed, due to Minecraft updating to LWJGL 3. Make sure you're
+   using the constants provided by the [`keys`] API, rather than hard-coding numerical values.
+
+   Related to this change, the numpad enter key now has a different key code to the enter key. You may need to adjust
+   your programs to handle both. (Note, the `keys.numpadEnter` constant was defined in pre-1.13 versions of CC, but the
+   `keys.enter` constant was queued when the key was pressed)
+
+ - Minecraft 1.13 removed the concept of item damage and block metadata (see ["The Flattening"][flattening]). As a
+   result `turtle.inspect` no longer provides block metadata, and `turtle.getItemDetail` no longer provides damage.
+
+   - Block states (`turtle.inspect().state`) should provide all the same information as block metadata, but in a much
+     more understandable format.
+
+   - Item and block names now represent a unique item type. For instance, wool is split into 16 separate items
+     (`minecraft:white_wool`, etc...) rather than a single `minecraft:wool` with each meta/damage value specifying the
+     colour.
+
+ - Custom ROMs are now provided using data packs rather than resource packs. This should mostly be a matter of renaming
+   the "assets" folder to "data", and placing it in "datapacks", but there are a couple of other gotchas to look out
+   for:
+
+   - Data packs [impose some restrictions on file names][legal_data_pack]. As a result, your programs and directories
+     must all be lower case.
+   - Due to how data packs are read by CC: Tweaked, you may need to use the `/reload` command to see changes to your
+     pack show up on the computer.
+
+   See [the example datapack][datapack-example] for how to get started.
+
+ - Turtles can now be waterlogged and move "through" water sources rather than breaking them.
+
+## CC: Tweaked 1.88.0 {#cc-1.88}
+ - Unlabelled computers and turtles now keep their ID when broken, meaning that unlabelled computers/items do not stack.
+
+## ComputerCraft 1.80pr1 {#cc-1.80}
+ - Programs run via `shell.run` are now started in their own isolated environment. This means globals set by programs
+   will not be accessible outside of this program.
+
+ - Programs containing `/` are looked up in the current directory and are no longer looked up on the path. For instance,
+   you can no longer type `turtle/excavate` to run `/rom/programs/turtle/excavate.lua`.
+
+[flattening]: https://minecraft.wiki.com/w/Java_Edition_1.13/Flattening
+[legal_data_pack]: https://minecraft.gamepedia.com/Tutorials/Creating_a_data_pack#Legal_characters
+[datapack-example]: https://github.com/cc-tweaked/datapack-example "An example datapack for CC: Tweaked"