Plot credits (sources, author, year...).
mf_credits( txt = "Source(s) & Author(s)", pos = "bottomleft", col, cex = 0.6, font = 3, bg = NA )
text of the credits, use '\n' to add line breaks
position, one of 'bottomleft', 'bottomright' or 'rightbottom'
cex of the credits
font of the credits
No return value, credits are displayed.