Drop D tuning

[Brett McCoy]

I have a score where I need to indicate Drop D tuning for the guitar
(6th string is tuned down to D). The typical way this is indicated in
guitar music is a (6) = D (circle with a 6 in it = D), placed at the
top left of page 1 under the title, but above a tempo mark. I am
relatively new to Lilypond and am unsure how to do this... any
suggestions?

-- Brett
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"In the rhythm of music a secret is hidden;
    If I were to divulge it, it would overturn the world."
               -- Jelaleddin Rumi