update readme

README.md

@@ -100,6 +100,32 @@ operation
 
 logging for cats, in ocaml: <https://git.lain.faith/haskal/xlog>
 
+### `pythonPackages.feedvalidator` or `feedvalidator`
+
+the W3C atom/RSS feed validator library, <https://github.com/w3c/feedvalidator>
+
+this package comes with an additional CLI bin, `feedvalidator`, which is a simple wrapper around the
+library that enables CLI usage
+
+usage
+```
+usage: feedvalidator [-h] [-b BASE] file
+
+W3C feedvalidator
+
+positional arguments:
+  file                  File to validate
+
+options:
+  -h, --help            show this help message and exit
+  -b BASE, --base BASE  Base URL of document
+```
+
+example
+```bash
+feedvalidator --base "https://my-base-url/atom.xml" path/to/atom.xml
+```
+
 ## lib documentation
 
 ### `fetchFromSteam`