Fixes python#12456

Things to consider:

Our current readme starts with "Got a question?". We could start with a short description instead, copying in our current description paragraph:

Add type annotations to your Python programs, and use mypy to type check them. Mypy is essentially a Python linter on steroids, and it can catch many programming errors by analyzing your program, without actually having to run it. Mypy has a powerful type system with features such as type inference, gradual typing, generics and union types."

Occasionally there is a relative link, like [CONTRIBUTING.md](CONTRIBUTING.md). Do those need to be changed?