Add capital M note

Fixes adafruit#23

Author: tannewt

adafruit_display_text/label.py

@@ -50,7 +50,8 @@ class Label(displayio.Group):
        glyph (if its one line), or the (number of lines * linespacing + M)/2. That is,
        it will try to have it be center-left as close as possible.
 
-       :param Font font: A font class that has ``get_bounding_box`` and ``get_glyph``
+       :param Font font: A font class that has ``get_bounding_box`` and ``get_glyph``.
+         Must include a capital M for measuring character size.
        :param str text: Text to display
        :param int max_glyphs: The largest quantity of glyphs we will display
        :param int color: Color of all text in RGB hex