docs/ci: note unit tests in README, gate release build to master

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ env:
 
 jobs:
   ci:
-    name: Check / Test / Build
+    name: Check / Test
     runs-on: ubuntu-latest
 
     steps:
@@ -38,5 +38,27 @@ jobs:
       - name: test
         run: cargo test
 
+  release-build:
+    name: Release build
+    runs-on: ubuntu-latest
+    if: github.ref == 'refs/heads/master' && github.event_name == 'push'
+    needs: ci
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Rust stable
+        uses: dtolnay/rust-toolchain@stable
+
+      - name: Cache cargo
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            target
+          key: ${{ runner.os }}-cargo-release-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-cargo-release-
+
       - name: build (release)
         run: cargo build --release -p presentation -p worker