Add README file listing flake8 violations defined

[peterjc]

I really like the idea of this plugin as a short term solution until black psf/black#26 is fixed.

I would encourage you to add a README file including an example showing what the flake8 error code prefix you are using is (recommended three letter, three number format, e.g. ABC001 to reduce the chance of duplicate codes between plugins). In your case, ISC as a prefix make sense to me.

[peterjc]

I see you are working on this in #2, and have included pip install flake8-implicit-str-concat which I forgot to suggest 👍

[peterjc]

I filed #3 on the wider issue of not using an alphanumeric flake8 violation code

[keisheiled]

now that #3 has landed, I'd love to see a PR with this added!