docs: add async setup example

Author: brettstack

README.md

@@ -37,6 +37,40 @@ const app = require('./app')
 exports.handler = serverlessExpress({ app })
 ```
 
+## Async setup Lambda handler
+
+If your application needs to perform some common bootstrap tasks such as connecting to a database before the request is forward to the API, you can use the following pattern (also available in [this example](https://github.com/vendia/serverless-express/blob/mainline/examples/basic-starter-api-gateway-v2/src/lambda-async-setup.js)):
+
+```js
+// lambda.js
+require('source-map-support/register')
+const serverlessExpress = require('@vendia/serverless-express')
+const app = require('./app')
+
+let serverlessExpressInstance
+
+function asyncTask () {
+  return new Promise((resolve) => {
+    setTimeout(() => resolve('connected to database'), 1000)
+  })
+}
+
+async function setup (event, context) {
+  const asyncValue = await asyncTask()
+  console.log(asyncValue)
+  serverlessExpressInstance = serverlessExpress({ app })
+  return serverlessExpressInstance(event, context)
+}
+
+function handler (event, context) {
+  if (serverlessExpressInstance) return serverlessExpressInstance(event, context)
+
+  return setup(event, context)
+}
+
+exports.handler = handler
+```
+
 ## 4.x
 
 1. Improved API - Simpler for end-user to use and configure.
@@ -208,4 +242,4 @@ On 11/30, the AWS Serverless Express library moved from AWS to [Vendia](https://
 We believe this is the best course of action to ensure that customers using this library get the best possible support in the future. To learn more about this move or become a maintainer of the new Serverless Express library, reach out to us through a GitHub issue on this repository. 
 
 Best,
-  The AWS Serverless team, Brett & the Vendia team
+  The AWS Serverless team, Brett & the Vendia team

examples/basic-starter-api-gateway-v2/scripts/local.js

@@ -13,6 +13,13 @@ const callback = (e, v) => {
   process.exit(0)
 }
 const server = lambdaFunction.handler(apiGatewayEvent, context, callback)
+server.then((v) => {
+  console.info(v)
+  process.exit(0)
+}).catch(e => {
+  console.error(e)
+  process.exit(0)
+})
 
 process.stdin.resume()
 

examples/basic-starter-api-gateway-v2/src/lambda-async-setup.js

@@ -0,0 +1,26 @@
+require('source-map-support/register')
+const serverlessExpress = require('@vendia/serverless-express')
+const app = require('./app')
+
+let serverlessExpressInstance
+
+function asyncTask () {
+  return new Promise((resolve) => {
+    setTimeout(() => resolve('connected to database'), 1000)
+  })
+}
+
+async function setup (event, context) {
+  const asyncValue = await asyncTask()
+  console.log(asyncValue)
+  serverlessExpressInstance = serverlessExpress({ app })
+  return serverlessExpressInstance(event, context)
+}
+
+function handler (event, context) {
+  if (serverlessExpressInstance) return serverlessExpressInstance(event, context)
+
+  return setup(event, context)
+}
+
+exports.handler = handler