doc: improved documentation for new parts, added todos (#12)

Co-authored-by: Jan Soukup <[email protected]>

Author: sladg

Diff for: README.md

@@ -4,6 +4,14 @@ This is a set of utils needed for deploying NextJS into AWS Lambda.
 It includes a wrapper for `next/server/image-optimizer` allowing to use S3.
 And includes CLI and custom server handler to integrate with ApiGw.
 
+- [NextJS Lambda Utils](#nextjs-lambda-utils)
+  - [Usage](#usage)
+  - [Sharp](#sharp)
+  - [Packaging](#packaging)
+    - [Server handler](#server-handler)
+    - [Static assets](#static-assets)
+- [TODO](#todo)
+
 ## Usage
 
 Create new lambda function with NODE_16 runtime in AWS.
@@ -45,8 +53,44 @@ const sharpLayer = new LayerVersion(this, 'SharpLayer', {
 })
 ```
 
-## Notes
+## Packaging
+
+In order to succefully deploy, you firstly need to include `target: 'standalone'` in your `next.config.js` setup.
+Make sure to use NextJS in version 12 or above so this is properly supported.
+
+Once target is set, you can go on and use your `next build` command as you normally would.
+To package everything, make sure to be in your project root folder and next folder `.next` and `public` exist. Packaging is done via NPM CLI command of `@slack/nextjs-lambda package`.
+
+It will create `next.out/` folder with 3 zip packages. One zip Lambda's code, one is dependencies layer and one is assets layer.
+
+- code zip: include all files generated by next that are required to run on Lambda behind ApiGateway. Original handler as well as new server handler are included. Use `handler.handler` for custom one or `server.handler` for original one.
+- dependencies layer: all transpilied `node_modules`. Next includes only used files, dramatically reducing overall size.
+- assets layer: your public folder together with generated assets. Keep in mind that to public refer file, you need to include it in `public/assets/` folder, not just in public. This limitation dramatically simplifies whole setup. This zip file is uploaded to S3, it's not included in Lambda code.
+
+### Server handler
+
+Custom wrapper around NextServer to allow for passing ApiGateway events into Next server.
+
+Cloudfront paths used:
+
+- `default`
+- `_next/data/*`
+
+### Static assets
+
+Next uses multiple directories to determine which file should be served. By default next provides us with list of routes for API/images/assets/pages. To simplify the process as much as possible, we are tapping into resulting paths.
+
+We are packaging those assets to simulate output structure and we are using S3 behind CloudFront to serve those files.
+Also, Image Handler is tapping into S3 to provide images, so correct folder structure is crucial.
+
+Cloudfront paths used:
+
+- `_next/*`
+- `assets/*`
 
-This is part of NextJS to Lambda deployment process. More info to follow.
+# TODO
 
-## @TODO: Add Server-handler description
+- Explain script used for packaging Next app,
+- Add CDK examples on how to set it up,
+- Export CDK contruct for simple plug-n-play use,
+- Use lib/index.ts as single entry and export all paths/functions from it (including zip paths).