docs: enhance the readme example to include multiple os, version, and archs

Author: aminya

README.md

@@ -74,24 +74,38 @@ jobs:
       - run: npm test
 ```
 
-Architecture:
-The architecture can be selected using `node-arch`. Values are `x86`, `x64`, `arm64`, `armv6l`, `armv7l`, `ppc64le`, `s390x` (not all of the architectures are available on all platforms).
+Operating Systems and Architecture:
+
+You can use any of the [supported operating systems](https://docs.github.com/en/actions/reference/virtual-environments-for-github-hosted-runners). The architecture can be selected using `node-arch`. Values are `x86`, `x64`, `arm64`, `armv6l`, `armv7l`, `ppc64le`, `s390x` (not all of the architectures are available on all platforms).
 ```yaml
 jobs:
   build:
-    runs-on: windows-latest
+    runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        node: [ '10', '12' ]
-        arch: ['x86', 'x64']
-    name: Node ${{ matrix.node }} on ${{ matrix.arch }}
+        os:
+          - ubuntu-latest
+          - macos-latest
+          - windows-latest
+        node_version:
+          - 10
+          - 12
+          - 14
+        node_arch:
+          - x64
+        # an extra windows-x86 run:
+        include:
+          - os: windows-2016
+            node_version: 12
+            node_arch: x86
+    name: Node ${{ matrix.node_version }} - ${{ matrix.node_arch }} on ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2
       - name: Setup node
         uses: actions/setup-node@v1
         with:
-          node-version: ${{ matrix.node }}
-          node-arch: ${{ matrix.arch }}
+          node-version: ${{ matrix.node_version }}
+          node-arch: ${{ matrix.node_arch }}
       - run: npm install
       - run: npm test
 ```