Improve README

[jamescooke]

• Describe Arrange Act Assert pattern.
• Describe Flake8
• Revert GitHub description field to "A Flake8 plugin that checks Python tests follow the Arrange-Act-Assert pattern"
• Explain what Flake8-AAA does and how it helps to write clear tests.
• Improve usage example - ideally show success and failure.
• Add command line example for running over all files in a test suite. find + xargs + python -m flake8_aaa
• Adjust home page of documentation to be unique and not reuse README.