Add Architecture only example

Author: aminya

README.md

@@ -74,9 +74,28 @@ jobs:
       - run: npm test
 ```
 
-Operating Systems and Architecture:
+Architecture:
+
+You can use any of the [supported operating systems](https://docs.github.com/en/actions/reference/virtual-environments-for-github-hosted-runners), and the compatible `architecture` can be selected using `architecture`. Values are `x86`, `x64`, `arm64`, `armv6l`, `armv7l`, `ppc64le`, `s390x` (not all of the architectures are available on all platforms).
+
+When using `architecture`, `node-version` must be provided as well.
+```yaml
+jobs:
+  build:
+    runs-on: windows-latest
+    name: Node sample
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-node@v1
+        with:
+          node-version: '12'
+          architecture: 'x64' # optional, x64 or x86. If not specified, x64 will be used by default
+      - run: npm install
+      - run: npm test
+```
+
+Multiple Operating Systems and Architectures:
 
-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: