DOC: NumPy-Financial should have a documented style guide

[Kai-Striega]

Issue with current documentation:

NumPy-Financial currently does not have a documented style guide

Idea or request for content:

Write a style guide for others to follow, the guide should be based of:

• PEP8 for general code, except the line length should be 88 characters
• numpydoc for documentation

Some further documents to consider:

• PEP8 and SciPy

In case of ambiguity, we follow the NumPy style guide