Each time src/translations.cpp is modified, we have to update the locale
patch. Using sed to patch dynamically should be handy.

Add new namespace 'cataclysmDDA', in which package builders, games, and
mods are listed.

Example:

let
  customMods = self: super: lib.recursiveUpdate super {
    soundpack.AwesomeSounds = cataclysmDDA.buildSoundPack { ... };
  };
in
cataclysm-dda.withMods (mods: with mods.extend customMods; [
  tileset.UndeadPeople
  soundpack.AwesomeSounds
])

'cataclysm-dda-git.overrideAttrs (_: { version = ...; src = fetchFromGitHub { ... }; })'
did not update VERSION make flag correctly.

With this change, one can override 'cataclysm-dda-git' correctly and
more easily:

cataclysm-dda-git.override { version = ...; rev = ...; sha256 = ...; }

Add description about `useXdgDir` flag

globstar (**) does not work in Makefile

cataclysmDDA: add modding interface