Update readme for arm64 pu/pu image (#398)

README.md

@@ -16,7 +16,9 @@ The base and SDK-specific images are considerably smaller than the combined `pul
 
 ## Build Matrix
 
-Each of the images described above (except the full `pulumi/pulumi` image) are built on a matrix of the following base images and platforms:
+`pulumi/pulumi` is built for amd64 and arm64, using Debian Bookworm (12) as the base image.
+
+Each of the other images described above are built on a matrix of the following base images and platforms:
 
 - [debian/debian:12-slim](https://github.com/debuerreotype/docker-debian-artifacts/blob/d99a48edaa18ad2bbb260c388b274c8c093f2d32/bullseye/slim/Dockerfile), (AKA "bookworm") tagged with the following suffixes:
   - `-debian-amd64`: Image manifest for the `linux/amd64` platform.