docs: add sha256 checksums for 18.2.8

Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
diff --git a/docs/relnotes/18.2.8.html b/docs/relnotes/18.2.8.html
index c93dcaa..be7643e 100644
--- a/docs/relnotes/18.2.8.html
+++ b/docs/relnotes/18.2.8.html
@@ -31,7 +31,8 @@
 
 <h2>SHA256 checksums</h2>
 <pre>
-TBD
+77512edc0a84e19c7131a0e2e5ebf1beaf1494dc4b71508fcc92d06d65f9f4f5  mesa-18.2.8.tar.gz
+1d2ed9fd435d86d95b7215b287258d3e6b1180293a36f688e5a2efc18298d863  mesa-18.2.8.tar.xz
 </pre>