title: How to rotate text in tm_credits
body: <p>I want to rotate the text of the tm_credits by 90 degrees, but tm_credits doesn't have an option to rotate.
  The default horizontal credits disturb the map so I want to place it along the left border of the map.</p>
  
  <p>An alternative of adding a simple text (i.e., not tm_credits) will also be fine for my needs.</p>
  
  <p>A simple example of what can be done:</p>
  
  <pre><code>data(World)
  tm_shape(World) +
  tm_fill("well_being", id="name", title="Well-being") +
  tm_format("World") +
  tm_credits("SOME CREDITS HERE", position=c("left", "bottom"))
  </code></pre>
  
  <p>Thanks</p>