docs(data): note that image and logo URLs keep their Commons licenses

image_url and logo_url now point only at Wikimedia Commons files. State that those files are not covered by the dataset's CC BY-SA 4.0 grant and how to credit them.

Refs #296

Author: Seungpyo1007

data/LICENSE.md

@@ -10,6 +10,14 @@ When you reuse the data, attribute it as "Data from TechAPI" with a link to
 same license. Each record also lists the sources it was compiled from in its
 `source_urls` field.
 
+## Images
+
+`image_url` and `logo_url` only link to files on Wikimedia Commons. Those files
+are not part of the dataset and this license does not cover them: each keeps
+the license shown on its Commons file page. Records with a photo also store its
+`image_license` and `image_attribution`; credit the author that way, or link
+to the file page, wherever you display one.
+
 This notice restates the license declared in the top-level `README.md`. The
 code in this repository — the validator, the site and the workflows — is
 licensed separately under MIT; see [`../LICENSE`](../LICENSE).