Add a GitHub action to push NuGet packages when a release is created

This commit won't actually push anything - but it will allow
us to test things before we *do* manually create a release.

Once testing is completed, we'll want to use a (suitably encrypted)
NuGet API key to push to NuGet.

Note that this initially assumes we want to release all packages at
the same time. We'll want to change it when we reach GA for 2.0.0.

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

.github/workflows/nuget.yml

@@ -0,0 +1,34 @@
+name: Push packages to NuGet
+
+on:
+  release:
+    types: [published]
+
+jobs:
+
+  release:
+    runs-on: ubuntu-18.04
+    env:
+      DOTNET_NOLOGO: true
+
+    steps:
+    - name: Check out our repo
+      uses: actions/checkout@v2
+
+    # Install the .NET Core 3.1 SDK for build and test
+    - name: Setup .NET Core 3.1
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: 3.1.x
+
+    - name: Build
+      run: |
+        dotnet build -c Release
+        dotnet test -c Release
+        mkdir nuget
+
+    - name: Push to NuGet
+      run: |
+        dotnet pack -c Release -o $PWD/nuget
+        # For now, just display the NuGet packages we'd push in reality.
+        ls nuget