Add more info on structuring config in pyproject.toml to the README

[RNKuhns]

It would be useful to include some additional documentation about how to structure the config file when using pyproject.toml.

I ran into some problems setting up the config file to ignore errors related to #78.

It looks like a pyproject.toml that includes something like what I've posted below, will work:

[tool.doc8]
max-line-length = 88
ignore = ["D004"]

However, based on the existing README, I initially didn't have the error quoted or in brackets. Not quoting the error codes leads to errors when reading the pyproject.toml file and leaving out the brackets from the ignore also appeared to make it so that doc8 didn't recognize what was being passed.

Can you confirm the above should work as expected to ignore error code D004? If so, should be an easy addition to the README to clarify for others.

[adam-grant-hendry]

@RNKuhns This doesn't seem to work for me (see my comment).