Branch organization for jekyll-theme-ed2

[karlstolley]

GitHub builds Jekyll sites from master branch from the repository specified the remote_theme property in _config.yml.

For that reason, master should always be the stable, released version of jekyll-theme-ed2 (once we get to our initial public-release versions, anyway).

And also for that reason, I propose that we maintain a next branch for jekyll-theme-ed2, which should be the base branch for all pull requests and other pre-release work.

Commits from next would then only appear on master as a part of a formal version release, whose initial procedures I've documented here: https://github.com/minicomp/jekyll-theme-ed2/wiki/Creating-releases

Assuming the idea of maintaining master and next branches this way sounds like a good idea, I will update those documented steps accordingly.