add new episode on branching and merging

[avallecam]

I think that more information on branching and merging using Git is required for a beginner. The collaboration workflow is heavily dependent on Git users being able to branch and make changes in parallel to collaborators. A lot of this information is already included in 14-supplemental-rstudio.html#create-a-branch but I think this should be moved into a standalone chapter with some more information.

Originally posted by @joshwlambert in #13 (comment)

[avallecam]

task can include

• create a new branch (from main or from an existing feature branch) https://git-scm.com/docs/git-checkout
• switch between branches https://git-scm.com/docs/git-switch
• delete a branch https://git-scm.com/docs/git-branch
• list all branches https://www.theserverside.com/blog/Coffee-Talk-Java-News-Stories-and-Opinions/delete-local-git-branch-origin-force-merge-all