fix license: CC0 -> CC-BY-4.0 (Glottolog requires attribution)

README.md

@@ -1,5 +1,5 @@
 ---
-license: cc0-1.0
+license: cc-by-4.0
 task_categories:
   - feature-extraction
 tags:
@@ -48,4 +48,4 @@ Applications include linguistic typology research, language documentation planni
 
 ## License
 
-CC0 1.0 (Public Domain)
+CC-BY-4.0 -- Glottolog data requires attribution. See LICENSE.md for details.