docs: guide on how to start renaming main branches in repos

[kajatiger]

Writing a guide on what repos to start renaming the main branch to "main". Please feel free to add to the lists or comment if some repo is not a good candidate to start with because there might be a hidden complexity in it.

[kajatiger]

Just found out that we in fact do load our own artsy gems into the gemfile by referring to the master branch:

  git: 'https://github.com/artsy/kinetic.git', branch: 'master'

and other examples. This needs to be considered as well!

[kajatiger]

[kajatiger]

[kajatiger]

[kajatiger]

[kajatiger]