ci update for multiple OS

Author: mihaj

.github/workflows/dotnet-core.yml

@@ -2,14 +2,17 @@ name: Build .NET Library
 
 on:
   push:
-    branches: [ main ]
   pull_request:
-    branches: [ main ]
+  release:
+    types:
+      - published
 
 jobs:
   build:
-
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ ubuntu-latest, windows-latest, macos-latest ]
 
     steps:
     - uses: actions/checkout@v2