Update usage-rmd-and-github.Rmd

[BjornBeckmann]

Hi, thanks for the super useful and accessible book. Just spotted a minor mistake here, to keep the intermediate .md files, need to add "keep_md: yes", not "keep_md: true". Best wishes, Bjorn

[jennybc]

My understanding is that both work and that keep_md: true is actually preferred. Can you elaborate on this? Meaning: did you experience a problem? Is there some resource that will correct or update my understanding?