[docs request] Adding example pre-commit config to README

[jamesbraza]

Can we add an example pre-commit hook to the README:

repos:
  - repo: https://github.com/grantjenks/blue
    rev: v0.9.1
    hooks:
      - id: blue

Source: #85 (comment)

Just will save a little bit of thought for developers trying to adopt blue via pre-commit