Update readme to show how to use the secret GITHUB_TOKEN

[warrenbuckley]

This updates the example with some inline comments to show that you need to use the GITHUB_TOKEN secret as the token which will need to be passed in for you to use & authenticate the GitHub API

[damccorm]

This seems helpful, thanks for the contribution.

[rowi1de]

Is this up-to-date? I'm getting Bad Credentials when doing

import * as core from '@actions/core';
import { context, GitHub } from '@actions/github'

const repoToken = core.getInput('repo-token', { required: true })
const client = new GitHub(repoToken)

      with:
        repo-token: ${{ secrets.GITHUB_TOKEN }}