docs: Point to apt.llvm.org for development snapshot packages

Signed-off-by: Kai Wasserbäch <kai@dev.carbon-project.org>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

docs/llvmpipe.html

@@ -50,6 +50,12 @@ It's the fastest software rasterizer for Mesa.
 <pre>
      aptitude install llvm-dev
 </pre>
+   <p>
+   If you want development snaptshot builds of LLVM for Debian and derived
+   distributions like Ubuntu, you can use the APT repository at <a
+   href="https://apt.llvm.org/" title="Debian Development packages for LLVM"
+   >apt.llvm.org</a>, which are maintained by Debian's LLVM maintainer.
+   </p>
    <p>
    For a RPM-based distribution do:
    </p>