Updated README for geoip-redirect package

packages/geoip-redirect/README.md

@@ -1,4 +1,40 @@
 # Geo-IP Redirect 
-This library provides a construct which creates a Lambda@Edge functions to perform GeoIP redirects.
 
-These functions are intended to be added to an existing Cloudfront distribution
+![TypeScript version](https://img.shields.io/github/package-json/dependency-version/aligent/cdk-constructs/dev/typescript?filename=packages/geoip-redirect/package.json&color=red) ![AWS CDK version](https://img.shields.io/github/package-json/dependency-version/aligent/cdk-constructs/dev/aws-cdk?filename=packages/geoip-redirect/package.json) ![NPM version](https://img.shields.io/npm/v/%40aligent%2Fcdk-geoip-redirect?color=green)
+
+## Overview
+
+This library provides a construct which creates a Lambda@Edge function to perform GeoIP redirects. The Aligent Esbuild CDK construct is used to bundle the handler function from `handler/redirect.ts`.
+
+These functions are intended to be added to an existing Cloudfront distribution. When the Lambda@Edge function is triggered, the function is passed the CloudFrontRequestEvent.
+
+The Lambda@Edge function will check if the viewer's country code matches any supported regions. The country code from each request is pulled from the `cloudfront-viewer-country` header:
+- if they match, they are redirected to `${redirectURL}${countryCode.toLowerCase()}${request.uri}`
+- if they do not match, they are redirected to `${redirectURL}${DEFAULT_REGION.toLowerCase()}${request.uri}`
+
+The `DEFAULT_REGION` is set as an option in the Lambda@Edge handler code
+
+## Diagram
+
+![geoip lambda@edge diagram](docs/geoip-redirect.drawio.png)
+
+## Usage and Default Geo-IP Redirect options
+### `redirectHost` (string)
+- Base hostname used for redirects
+
+### `supportedRegionsExpression` (string)
+- Case-sensitive regular expression matching cloudfront-viewer-country
+
+### `defaultRegion` (string)
+- Fallback region code when viewer's country does not match the supported pattern
+
+## Example code
+When defining the Lambda@Edge function, this construct serves one default host to redirect too:
+```
+define: {
+                "process.env.REDIRECT_HOST": options.redirectHost,
+                "process.env.SUPPORTED_REGIONS":
+                  options.supportedRegionsExpression,
+                "process.env.DEFAULT_REGION": options.defaultRegion,
+              },
+```