Update yaml in README.md, fix deprecations (#24)

Co-authored-by: Davide Violante <[email protected]>

Author: xeeo

README.md

@@ -14,26 +14,34 @@ An example workflow to deploy a project with serverless:
 
 
 ```yaml
+name: Deploy master branch
+
 on:
   push:
     branches:
       - master
-name: Deploy master branch
+
 jobs:
   deploy:
     name: deploy
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node-version: [12.x]
     steps:
-    - uses: actions/checkout@master
-    - name: npm install
-      run: npm install
+    - uses: actions/checkout@v2
+    - name: Use Node.js ${{ matrix.node-version }}
+      uses: actions/setup-node@v1
+      with:
+        node-version: ${{ matrix.node-version }}
+    - run: npm ci
     - name: serverless deploy
       uses: serverless/github-action@master
       with:
         args: deploy
       env:
         SERVERLESS_ACCESS_KEY: ${{ secrets.SERVERLESS_ACCESS_KEY }}
-        # or if using AWS creds directly
+        # or if using AWS credentials directly
         # AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}
         # AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
 ```