Add SBOM extraction instructions

Author: thomasleplus

README.md

@@ -56,6 +56,17 @@ To know more command-line options of `jq`:
 docker run --rm --net=none leplusorg/json jq -h
 ```
 
+## Software Bill of Materials (SBOM)
+
+To get the SBOM for the latest image (in SPDX JSON format), use the
+following command:
+
+```bash
+docker buildx imagetools inspect leplusorg/json --format '{{ json (index .SBOM "linux/amd64").SPDX }}'
+```
+
+Replace `linux/amd64` by the desired platform (`linux/amd64`, `linux/arm64` etc.).
+
 ## Request new tool
 
 Please use [this link](https://github.com/leplusorg/docker-json/issues/new?assignees=thomasleplus&labels=enhancement&template=feature_request.md&title=%5BFEAT%5D) (GitHub account required) to request that a new tool be added to the image. I am always interested in adding new capabilities to these images.