Mention a dependency to libfreetype6.

Libfreetype6 is essentially a dependency to compile Pillow with
libfreetype support. Without this library, the activity graphs are
broken and the following traceback appears in the logs:
  Traceback (most recent call last):
    File "/var/www/weblate-venv/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 112, in get_response
      response = wrapped_callback(request, *callback_args, **callback_kwargs)
    File "/var/www/weblate/weblate/trans/views/charts.py", line 122, in monthly_activity
      return render_activity(activity)
    File "/var/www/weblate/weblate/trans/views/charts.py", line 52, in render_activity
      font = get_font(11)
    File "/var/www/weblate/weblate/trans/fonts.py", line 695, in get_font
      size
    File "/var/www/weblate-venv/local/lib/python2.7/site-packages/PIL/ImageFont.py", line 228, in truetype
      return FreeTypeFont(font, size, index, encoding)
    File "/var/www/weblate-venv/local/lib/python2.7/site-packages/PIL/ImageFont.py", line 131, in __init__
      self.font = core.getfont(font, size, index, encoding)
    File "/var/www/weblate-venv/local/lib/python2.7/site-packages/PIL/ImageFont.py", line 42, in __getattr__
      raise ImportError("The _imagingft C module is not installed")
  ImportError: The _imagingft C module is not installed
Merge pull request #495 from nblock/documentation/mention-libfreetype

Mention a dependency to libfreetype6.