Improve documentation

[dgtlmoon]

Hello and thank you for the project, but it would really help people to contribute to it if the documentation was a little better :)

The USAGE example could be a little more clearer I think.

[dgtlmoon]

Thanks! yeah, it's a small detail, but it helps alot :)

[tonthon]

Since we're missing some docs.
I'll say it here
There are some hardcoded header tests regarding the date format that replace the datebase
https://github.com/majerteam/sylk_parser/blob/master/sylk_parser/sylk.py#L165

So depending the files you want to parse, you may encounter some date translation problems