nixos/amazon-image: Drop create-amis.sh script and update docs

We replaced this with automation in https://github.com/NixOS/amis

Also add some docs how you can build the image yourself

nixos/maintainers/scripts/ec2/README.md

@@ -1,7 +1,36 @@
 # Amazon images
 
-* The `create-amis.sh` script will be replaced by https://github.com/NixOS/amis which will regularly upload AMIs per NixOS channel bump.
+AMIs are regularly uploaded from Hydra. This automation lives in
+https://github.com/NixOS/amis
 
-* @arianvp is planning to drop zfs support
-* @arianvp is planning to rewrite the image builder to use the repart-based image builder.
 
+## How to upload an AMI for testing
+
+If you want to upload an AMI from changes in a local nixpkgs checkout.
+
+```bash
+nix-build nixos/release.nix -A amazonImage
+
+export AWS_REGION=us-west-2
+export AWS_PROFILE=my-profile
+nix run nixpkgs#upload-ami -- --image-info ./result/nix-support/image-info.json
+```
+
+## How to build your own NixOS config into an AMI
+
+I suggest looking at https://github.com/nix-community/nixos-generators for a user-friendly interface.
+
+```bash
+nixos-generate -c ./my-config.nix -f amazon
+
+export AWS_REGION=us-west-2
+export AWS_PROFILE=my-profile
+nix run github:NixOS/amis#upload-ami -- --image-info ./result/nix-support/image-info.json
+```
+
+## Roadmap
+
+* @arianvp is planning to drop zfs support unless someone else picks it up
+* @arianvp is planning to rewrite the image builder to use the repart-based image builder.
+* @arianvp is planning to perhaps rewrite `upload-ami` to use coldnsap
+* @arianvp is planning to move `upload-ami` tooling into nixpkgs once it has stabilized. And only keep the Github Action in separate repo