Add a footnote reference following #55

docs/README.md

@@ -4,7 +4,7 @@
 
 `alto2txt` converts `XML` `ALTO`/`METS` Optical Character Recognition (OCR) scans into plaintext files with minimal metadata.
 
-`ALTO` and `METS` are industry standards targeting newspaper digitization used by hundreds of modern, large-scale newspaper digitization projects. One text file is output per article, each complemented by one `XML` metadata file.
+`ALTO` and `METS` are industry standards maintained by the US Library of Congress targeting newspaper digitization used by hundreds of modern, large-scale newspaper digitization projects. One text file is output per article, each complemented by one `XML` metadata file[^CHNC].
 
 [`METS` (Metadata Encoding and Transmission Standard)](http://www.loc.gov/standards/mets/) is a standard for encoding descriptive, administrative, and structural metadata regarding objects within a digital library, expressed in `XML`. [`ALTO` (Analyzed Layout and Text Objects)](https://www.loc.gov/standards/alto/) is an [`XML schema`](https://en.wikipedia.org/wiki/XML_schema) for technical metadata describing the layout and content of text resources such as book or newspaper pages. `ALTO` is often used in combination with `METS` but can also be used independently. Details of the `ALTO` schema are avilable at https://github.com/altoxml/schema.
 
@@ -147,4 +147,6 @@ Quality assurance is performed to check for:
 * Check and ensure that articles that span multiple pages are pulled into a single article file.
 * Smarter handling of articles spanning multiple pages.
 
+[^CHNC]: For a more detailed description see: https://www.coloradohistoricnewspapers.org/forum/what-is-metsalto/
+
 > Last updated 2022-12-06