Add github actions workflow stub.

This apparently needs to exist in the main branch for Github Actions to
recognize that it exists.

.github/workflows/build.yml

@@ -0,0 +1,29 @@
+name: Build SynchronousAudioRouter
+on:
+  workflow_dispatch:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+permissions:
+  contents: read
+  pull-requests: read
+concurrency:
+  group: "${{ github.workflow }} @ ${{ github.event.pull_request.head.label || github.head_ref || github.ref }}"
+  cancel-in-progress: true
+jobs:
+  build:
+    runs-on: windows-latest
+
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Setup MSBuild
+        uses: microsoft/setup-msbuild@v1.1.3
+        with:
+          vs-version: "[17.0,18.0)"
+          msbuild-architecture: x64
+
+      - name: Build C++ Project
+        working-directory: ${{env.GITHUB_WORKSPACE}}
+        run: msbuild SynchronousAudioRouter.sln /t:Build /p:Configuration=Release /p:Platform=x64 /m /verbosity:minimal