docs: specify node.js version in Github Actions example (#679)

Author: j0nas

README.md

@@ -19,6 +19,8 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-node@v1
+        with:
+          node-version: 14
       - run: npm install && npm install -g @lhci/[email protected]
       - run: npm run build
       - run: lhci autorun