Update readme.md

Author: cliftonc

readme.md

@@ -30,6 +30,12 @@ It looks like this after it has run:
 
 <img width="1451" alt="Screenshot 2021-12-19 at 12 55 39" src="https://user-images.githubusercontent.com/239305/146673989-01187d53-d2fd-42ba-9968-31442b8cc92d.png">
 
+## Service Descriptor Format
+
+This action expects each of your repositories to have a descriptor file format in the root of the repo in the form of a Backstage `catalog-info` file.  This is because we are testing this approach against using Backstage directly, and wanted to leverage a format that perhaps has a chance of becoming a defacto standard.  It does not currently map all fields from Backstage, but if you look at the code you can see what it does map.  I may add a config option to allow mapping to be more dynamic in future (a good PR!). 
+
+Information on the format: https://backstage.io/docs/features/software-catalog/descriptor-format
+
 ## Usage
 
 This is typically deployed as a scheduled action: