Readme: Use ignoreErrors to use all but a few strict rules

[leongersen]

I'm using this strategy to enable strict rules in an existing project. While some rule have errors that could be fixed or added to the baseline, others are more fundamental to the project structure. I'd like to keep using the extension installer, so instead of manually configuring the individual rules, I'm ignoring some errors using ignoreErrors.

[ondrejmirtes]

The first two can be ignored with:

parameters:
    reportMaybesInMethodSignatures: false

I plan to do a much nicer way to disable specific strict rules, stay tuned :)

[ondrejmirtes]

I've already merged #151 but thanks :)