child-the-end-of-repo-setup.Rmd

Now just "lather, rinse, repeat".
Do work somewhere: locally or on GitHub.
Commit it.
Push it or pull it, depending on where you did the work, but get local and remote "synced up".
Repeat.

Note that in general (and especially in future when collaborating with other developers) you will usually need to pull changes from the remote (GitHub) before pushing the local changes you have made.
For this reason, it's a good idea to try and get into the habit of pulling before you attempt to push.