Add npm publish workflow for psp client

.github/workflows/publish.yml

@@ -0,0 +1,23 @@
+name: publish
+
+on:
+  release:
+    types: [created]
+
+jobs:
+  publish-npm:
+    name: publish to npm
+    runs-on: ubuntu-latest
+    steps:
+
+      - name: configure wombat dressing repository
+        uses: actions/setup-node@v1
+        with:
+          node-version: '12.x'
+          registry-url: https://wombat-dressing-room.appspot.com
+          scope: '@google-pay'
+
+      - name: publish @google-pay/psp-client to npm (via wombat dressing)
+        run: cd google-pay-psp-client && npm publish --access=public
+        env:
+          NODE_AUTH_TOKEN: ${{secrets.TOKEN_PSP_CLIENT}}