docs: add sha256 checksums for 17.3.9

Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
diff --git a/docs/relnotes/17.3.9.html b/docs/relnotes/17.3.9.html
index 8d528d9..5075737 100644
--- a/docs/relnotes/17.3.9.html
+++ b/docs/relnotes/17.3.9.html
@@ -31,7 +31,8 @@
 
 <h2>SHA256 checksums</h2>
 <pre>
-TBD
+4d625f65a1ff4cd8cfeb39e38f047507c6dea047502a0d53113c96f54588f340  mesa-17.3.9.tar.gz
+c5beb5fc05f0e0c294fefe1a393ee118cb67e27a4dca417d77c297f7d4b6e479  mesa-17.3.9.tar.xz
 </pre>