docs/relnotes: Add sha256 sums for mesa 18.1.1

docs/relnotes/18.1.1.html

@@ -31,7 +31,8 @@ Compatibility contexts may report a lower version depending on each driver.
 
 <h2>SHA256 checksums</h2>
 <pre>
-TBD
+366a35f7530a016f2a8284fb0ee5759eeb216b4d6fa47f0e96b89ad2e43faf96  mesa-18.1.1.tar.gz
+d3312a2ede5aac14a47476b208b8e3a401367838330197c4588ab8ad420d7781  mesa-18.1.1.tar.xz
 </pre>