Expand documentation on coverage report setup

[gomain]

Please expand the coverage report section of the README to include minimal steps to set up coverage reporting. Such that people with lesser knowledge of babel and loaders can follow.

Included discussion was based on my personal experience. For context I use pnpm with workspaces via rush.

Discussed in #71 (comment)

So I'v finally got it working.

1. Install "babel-register-esm", save dev
2. In ./jest.config.mjs

   collectCoverage: true,
   coverageProvider: "babel",
   coverageReporters: ["text"],

3. Add file ./babel.config.json

   {
     "plugins": ["babel-plugin-istanbul"]
   }

4. Run tests with NODE_OPTIONS='--loader=babel-register-esm' jest