hack/rhcos: add README

Documents how to run the rhcos stream marketplace populator program.

Author: patrickdillon

hack/rhcos/README.md

@@ -0,0 +1,25 @@
+# Populating the RHCOS Stream Marketplace Extension
+
+This tool is used to reach out to cloud sdks and populate the
+data/data/coreos/marketplace-rhcos.json file. That file represents
+the mareketplace extension to the rhcos stream and is merged into
+the stream.
+
+To run the program:
+
+```shell
+go run -mod=vendor ./hack/rhcos/populate-marketplace-imagestream.go
+```
+
+The program will find marketplace images based on the version of the
+RHCOS stream. The version can be overriden with the
+`STREAM_RELEASE_OVERRIDE` variable. This is useful if you are working
+on the main branch, where up-to-date images are not available. For
+example, looking up Azure images requires knowing the X.Y version
+to populate the offer, so when working on the main branch (4.20),
+it is necessary to run the following command to correctly populate
+the stream:
+
+```shell
+STREAM_RELEASE_OVERRIDE=4.19 go run -mod=vendor ./hack/rhcos/populate-marketplace-imagestream.go
+```