message: chore: update readme usage notice heading
diff: <img src="https://raw.githubusercontent.com/pact-foundation/pact-logo/master/media/logo-black.png" width="200">
  -| :information_source: |
  -|-----------------------------------------|
  +| :information_source: Usage notice |
  +|:-----------------------------------------|
  | This is a core library, designed for use in the bowels of another package. Unless you are wanting to develop tools for the pact ecosystem, you almost certainly want to install [`@pact-foundation/pact`](https://github.com/pact-foundation/pact-js/) instead|
  ![Build and test](https://github.com/pact-foundation/pact-js-core/workflows/Build%20and%20test/badge.svg)
  