docs: add instructions to specify LLVM version for basic testing

docs: add instructions to specify LLVM version for basic testing

The "Perform basic testing" and "Use the release.sh script from xorg
util-modular" sections provide some instructions to do so. We add now
some comments in order to use a recent enough LLVM version to run
dist/distcheck and the automake generated binaries.

v2: Suggested the need to define LLVM_CONFIG also before running the
    release.sh script.

Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>