Add an example of excluding folders from yapf to the documentation

[pksol]

I wanted to ignore my tests folder, so I had to hack a bit till I got to this way:

args: --verbose --exclude '**/tests/**' (the trivial way would be to send just 'tests/**' as a parameter, but it assumes that the execution is inside the repository).

Can you include it to the documentation so people can use it quickly? I didn't do a pull request in case you wanted to solve it in a different way.

[AlexanderMelde]

Thank you for the idea and explanation, i will add it!

[AlexanderMelde]

Implemented in commit b65bdf0 .