Update README.md with --custom-config

[to-long]

Related to #503
I want to update Readme.md to introduce this option to other user

[js2me]

@BrianTo176 hello, thanks for this pull request! Please switch target branch to the next

[smorimoto]

@js2me I just changed the target branch on behalf of @BrianTo176 🙃

[to-long]

thanks @smorimoto

[shrpne]

Sorry, but it is not clear what exact data should be in the config, config should export an object with options fields?

Like:

export default {
  primitiveTypeConstructs: (constructs) => ({
      ...constructs,
      string: {
        'date-time': 'Date'
      }
  }),
}