docs: Add SHA256 sum for 19.2.1

docs/relnotes/19.2.1.html

@@ -35,7 +35,7 @@ depends on the particular driver being used.
 
 <h2>SHA256 checksum</h2>
 <pre>
-TBD.
+    4cc53ca1a8d12c6ff0e5ea44a5213c05c88447ab50d7e28bb350cd29199f01e9  mesa-19.2.1.tar.xz
 </pre>