docs(readme): add considerations for deploying with vercel (#49)

Author: oscard0m

README.md

@@ -22,6 +22,12 @@ Follow the instructions to register a new GitHub app.
 
 The app is continuously deployed using [Vercel's GitHub app](https://github.com/apps/vercel).
 
+### Considerations
+- Make sure you configure [the environment variables for your GitHub App](https://probot.github.io/docs/configuration/) in Vercel. You can read more about how to do it in [their docs](https://vercel.com/docs/concepts/projects/environment-variables).
+- Vercel [expects to find your lambda functions under `/api` folder]([url](https://vercel.com/docs/concepts/functions/serverless-functions#deploying-serverless-functions)). Make sure your functions are placed there and double check Vercel detected your Lambda Functions during the deployment process by checking the logs:
+
+![image](https://user-images.githubusercontent.com/2574275/187179364-b0019f95-be41-462a-97d5-facf4de39095.png)
+
 ## How it works
 
 The [api/github/webhooks/index.js](api/github/webhooks/index.js) file is handling requests to `POST /api/github/webhooks`, make sure to configure your GitHub App registration's webhook URL accordingly.