document and check for git lfs prior to build (#5)

Git LFS is required for this repo; add some documentation and a build-time check that LFS has been used correctly

Co-authored-by: Bryton Hall <[email protected]>

CONTRIBUTING.md

@@ -6,6 +6,8 @@ This project uses the [nix](https://nixos.org/download.html) package manager.
 
 ## build
 
+> **WARN**: git LFS must be enabled to resolve various `.a` binary files
+
 Enter a development environment with
 
 ```sh

flake.nix

@@ -116,6 +116,12 @@
                 ffmpeg
                 xxd
               ];
+              postUnpack = ''
+                if [ "$(head -c 7 $(git lfs ls-files --name-only | head -1))" =! "version" ]; then
+                  echo "ERROR: run 'git lfs pull' to resolve binary files"
+                  exit 1
+                fi
+              '';
               buildPhase = ''
                 find ./config/res/spoken -maxdepth 1 -mindepth 1 -type d -exec sh -c 'make -j LANGUAGE=$(basename {})' \;
               '';