Added .github folder with workflow for github actions.

- Setting up continuous integration based on the workflow from Raylib-cs.

Author: ChrisDill

.github/workflows/dotnetcore.yml

@@ -0,0 +1,20 @@
+name: .NET Core
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    name: Build on dotnet using ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest, macOS-latest]
+
+    steps:
+      - uses: actions/checkout@v1
+      - name: Setup .NET Core
+        uses: actions/setup-dotnet@v1
+        with:
+          dotnet-version: 3.1.100
+      - name: Build with dotnet
+        run: dotnet build --framework=netcoreapp3.1 -c Release