add github action for CI (#98)

* add ci action

Author: Meir017

.github/workflows/ci.yml

@@ -0,0 +1,17 @@
+name: CI
+on: [pull_request]
+jobs:
+  build:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - name: Setup .NET 3
+        uses: actions/setup-dotnet@v1
+        with:
+          dotnet-version: '3.1.x'
+      - run: dotnet test src/FluentAssertions.Analyzers.sln --configuration Release  --filter 'TestCategory=Completed'