Readme update to precise instructions to replace key and secret

[chris60201]

Instructions

1. PR target branch should be against development
2. PR title name should follow this format: https:/mParticle/mparticle-workflows/blob/stable/.github/workflows/pr-title-check.yml
3. PR branch prefix should follow this format: https:/mParticle/mparticle-workflows/blob/stable/.github/workflows/pr-branch-check-name.yml

Summary

Read me update to precise instructions already written in mParticle docs on how to replace key and secret

Testing Plan

• {explain how this has been tested, and what additional testing should be done}

Reference Issue

• Closes https://go.mparticle.com/work/REPLACEME