docs: add sha256 checksums for 17.3.6

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit b00880973e)

docs/relnotes/17.3.6.html

@@ -31,7 +31,9 @@ because compatibility contexts are not supported.
 
 <h2>SHA256 checksums</h2>
 <pre>
-TBD
+d5e10ea3f0d11b06d2b0b235bba372a04278c39bc0e712090bda1f61842db188  mesa-17.3.6.tar.gz
+e5915680d44ac9d05defdec529db7459ac9edd441c9845266eff2e2d3e57fbf8  mesa-17.3.6.tar.xz
+</pre>
 
 
 <h2>New features</h2>