point out font dependencies

[relleums]

PIL expects fonts to be installed. If the fonts can not be found on the system, the 'time' and 'status' images do not have any red text and remain black. This happened lately when la_palma_overview was moved to newdata which did not have the fonts in question.

stdout on newdata:

2017-08-22 07:57:23|ERROR|la_palma_overview|Failed to create clock image
Traceback (most recent call last):
  File "/home/fact/anaconda3/lib/python3.5/site-packages/la_palma_overview/__init__.py", line 284, in save_image
    imgs.append(clock2img(cfg['img']['rows'], cfg['img']['cols']))
  File "/home/fact/anaconda3/lib/python3.5/site-packages/la_palma_overview/__init__.py", line 49, in clock2img
    font_time = ImageFont.truetype('DejaVuSansMono.ttf', size=120)
  File "/home/fact/anaconda3/lib/python3.5/site-packages/PIL/ImageFont.py", line 239, in truetype
    return FreeTypeFont(font, size, index, encoding)
  File "/home/fact/anaconda3/lib/python3.5/site-packages/PIL/ImageFont.py", line 128, in __init__
    self.font = core.getfont(font, size, index, encoding)
OSError: cannot open resource

At the moment I do not know how to fix this to be pip installable. I have the feeling that we might be able to bring the font file ourselves in the package resources and then tell PIL to use this font file. But at the moment it should at least be mentioned in the README.md