Add note for Github Pages when initial branch is not named master

[BareTuna]

For a little bit I was wondering why my deployment wasn't working, even though I changed very little. What happened was I deleted the .git folder and re-initialized the repository to clear commit history, but my default initial branch name is main. That broke the workflow because the pages.yml file by default uses the master branch

on:
  push:
    branches:
      - master

So this PR just adds a note in the README

I think it might also be worth considering adding a couple common branch names in the pages.yml file and have the user uncomment them (my eyes skipped over master since it looks similar to main)

# By default, runs if you push to master. keeps your deployed app in sync with master branch.
# If you renamed your initial branch name to something else, uncomment or change it below
on:
  push:
    branches:
      - master
#     - main
#     - trunk

[emilk]

Thanks!