Add initial CI

.github/workflows/ci.yaml

@@ -0,0 +1,30 @@
+name: Build/Test
+
+on:
+- push
+- pull_request
+
+jobs:
+  build:
+    name: .NET on ${{ matrix.os }}
+
+    strategy:
+      matrix:
+        os:
+        - ubuntu-latest
+        - macos-latest
+        - windows-latest
+        configuration:
+        - debug
+        - release
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+    - uses: actions/checkout@v2.3.4
+
+    - name: Setup .NET
+      uses: actions/setup-dotnet@v1.8.2
+
+    - name: Build Solution
+      run: dotnet build -c ${{ matrix.configuration }} src/shamir.sln