Create dotnet-format.yml

Add github action to check if new/touched file is formatted.

.github/workflows/dotnet-format.yml

@@ -0,0 +1,20 @@
+name: Format check on pull request
+on: pull_request
+
+jobs:
+  dotnet-format:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v2
+
+      - name: Add dotnet-format problem matcher
+        uses: xt0rted/dotnet-format-problem-matcher@v1
+
+      - name: Restore dotnet tools
+        uses: xt0rted/dotnet-tool-restore@v1
+
+      - name: Run dotnet format
+        uses: xt0rted/dotnet-format@v1
+        with:
+          only-changed-files: true