I get asked why we can't use the GitHub merge button at onboardings from time to time and I never know what issues with it are real and what I merely imagine.

onboarding.md currently says:

• Please never use GitHub's green "Merge Pull Request" button.
  • If you do, please force-push removing the merge.

And that's it. One or two sentences explaining the deal-breaker incompatibilities with our workflow would be great. @cjihrig? Someone else?