Minor changes to documentation

[MarkTheGoose]

Excellent and useful tool, thank you.

In README.md, you might consider making the statement:

witty = wittyPy.WittyPi('path/to/wittyPi/Software')

slightly clearer, it caught me out until I figurered that it's a relative or absolute path statement. Mine ended up as

witty = wittyPy.WittyPi('/home/pi/wittyPi/')

which needed the '/' on both ends. I assume that if I was using the home directory '/home/pi' to run Python, then I could just use 'wittyPi/' as the path, but still with the '/' on the end.
Also, it would be good to make it clear that it's for Python3, as Raspbian still defaults to Python2.7

Finally, on your Guide page: http://docs.elpunkt.eu/wittyPy/guide.html
You could again be clearer with the path statement (as above) and there is an error in line 12

The example you give is:

print(witty.next_startup)

which should be:

print(witty.next_start)

[benjaminfuchs]

I would suggest to use "os.path.join" here instead of "+":
https://github.com/elpunkt/wittyPy/blob/master/wittyPy/wittyPy.py#L45

Than it works with and without "/" at the end.