update readme

Author: patinthehat

README.md

@@ -99,6 +99,13 @@ If the optional field `verify` is set to `false`, the application will not attem
 
 If the optional `checksum-url` field is not specified, the application will attempt to fetch the checksum file from the same location as the included file, but with the `.sha256` extension appended to the filename.  For example, if the included file is `gh:permafrost-dev/stackup/main/templates/remote-includes/containers.yaml`, the checksum file will be fetched from `gh:permafrost-dev/stackup/main/templates/remote-includes/containers.yaml.sha256`.
 
+Alternatively, a single `checksums.sha256.txt` or `checksums.sha512.txt` file can exist instead of separate `*.sha256/sha512` files for each file in the `includes` section.  The file should contain a list of checksums for each included file, one per line, in the format `checksum filename` (where `filename` is the base filename only). This is the format generated by the `sha256sum/sha512sum` command-line utilities.  For example:
+
+```text
+9e0d9fea90950908c356734df89bfdff4984de4a6143fe32c404cfbc91984fb7  containers.yaml
+69b87009a87e38e5470191a9e40c441ce963fb4cf260fd44cf5f032b9566454a  laravel.yaml
+```
+
 Valid algorithms are `sha256` or `sha512`, and checksum files may be generated with the `sha256sum` or `sha512sum` command line utilities.
 
 ### Configuration: Preconditions