Add a first GitHub action to build PRs and the main branch

(We can add a badge later, and potentially remove the AppVeyor build
if we want.)

Signed-off-by: Jon Skeet <jonskeet@google.com>

.github/workflows/build.yml

@@ -0,0 +1,35 @@
+name: Build
+
+on:
+  push:
+    branches:
+    - master
+  pull_request:
+
+jobs:
+
+  build:
+    runs-on: ubuntu-18.04
+    env:
+      DOTNET_NOLOGO: true
+
+    steps:
+    - name: Check out our repo
+      uses: actions/checkout@v2
+
+    # We need just the .NET Core 2.1 runtime for testing    
+    - name: Setup .NET Core 2.1
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: 2.1.x
+
+    # Build with .NET Core 3.1 SDK
+    - name: Setup .NET Core 3.1
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: 3.1.x
+
+    - name: Build
+      run: |
+        dotnet build
+        dotnet test