Use the default flake8 ignores

default-flake8-ignores

1576352044

Flake8 provides a way to ignore PEP8 rules through its `--ignore` and
`--extend-ignore` flags.  The former overwrites a default list of errors
and warnings, including [W503](https://lintlyci.github.io/Flake8Rules/rules/W503.html)
and [W504](https://lintlyci.github.io/Flake8Rules/rules/W504.html) which are
mutually incompatible.  Therefore, it's easier to just use `--extend-ignore` and
not use `--ignore`.

Add a note about default flake8 ignores
Add a note about default flake8 ignores
