readme: how to invalidate cdn cache

Author: sorah

README.md

@@ -12,7 +12,7 @@ Deployments are automatically performed on GitHub Actions on `master` branch aft
 - App Runner (deprecated): [arn:aws:apprunner:us-west-2:005216166247:service/rko-router/2c9219ae5e14411baaf46fa932f33025](https://us-west-2.console.aws.amazon.com/apprunner/home?region=us-west-2#/services/dashboard?service_arn=arn%3Aaws%3Aapprunner%3Aus-west-2%3A005216166247%3Aservice%2Frko-router%2F2c9219ae5e14411baaf46fa932f33025&active_tab=logs)
 - CloudFront: [arn:aws:cloudfront::005216166247:distribution/E2WEWQCYU12GVD](https://us-east-1.console.aws.amazon.com/cloudfront/v3/home?region=ap-northeast-1#/distributions/E2WEWQCYU12GVD)
 
-All resources except deployment is managed under Terraform (`./tf`).
+All resources except deployment is managed under Terraform [./tf](./tf).
 
 ### Domains
 
@@ -22,6 +22,12 @@ Due to the quota of custom domains per App Runner service and the lambda functio
 
 These custom headers are assigned at CloudFront function (viewer-request) and implementation is at [./tf/cf_functions/src/viewreq.ts](./tf/cf_functions/src/viewreq.ts).
 
+### Cache invalidation
+
+While rubykaigi.org deployment automatically invalidates CloudFront cache on each deployment, rko-router does not.
+
+You need to manually invalidate the cache: https://rubykaigi.esa.io/posts/1241#%E3%82%AD%E3%83%A3%E3%83%83%E3%82%B7%E3%83%A5
+
 ## Run locally
 
 ```