Added GitHub workflow action

Author: antonyvorontsov

.github/workflows/dotnetcore.yml

@@ -0,0 +1,21 @@
+name: .NET Core workflow
+
+on: [push]
+
+jobs:
+  test:
+
+    runs-on: ${{ matrix.os }}
+    strategy:
+          matrix:
+            os: [macOS-latest, ubuntu-latest, windows-latest]
+    steps:
+    - uses: actions/checkout@v2
+    - name: Setup .NET Core
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: 3.1.100
+    - name: Build
+      run: dotnet build --configuration Release
+    - name: Run tests
+      run: dotnet test --configuration Release