Update documentation for github drag and drop

[zkamvar]

Back in February, GitHub announced that it is now possible to drag and drop files into the repositories:

Blog here: https://github.com/blog/2105-upload-files-to-your-repositories
Help page here: https://help.github.com/articles/adding-a-file-to-a-repository/

This could make things easier for contributors as the steps would be:

1. Update fork
2. Create new branch
3. Drag and drop your files + commit on GitHub
4. Pull request

A few things that might make this confusing:

• users would have to switch between Rstudio and GitHub
• synching a branch from your fork to your local repo is not well documented (currently)

Despite these setbacks, I think it would be a good idea to include these links and later do a quick video demonstration (~5 minutes).

[hlapp]

Are you sure this contains the details for eb7c52a, or did you mean to reference a different issue?

Actually, I notice that the commit references 176, not this one. I'm confused now why it's showing up here.

[zkamvar]

It's coming up here because I accidentally made a commit comment that referenced this issue and then deleted it.

[zkamvar]

It turns out that updating a branch created on github in git2r is really easy 😮:

fetch(repo, "origin")
checkout(repo, "newbranch")

This would be the process used when the user wants to make changes on his/her computer.