Add simple CI workflow

.github/workflows/build.yml

@@ -0,0 +1,19 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Install Nix
+        uses: cachix/install-nix-action@v16
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+
+      - name: Build the book
+        run: nix-build release.nix