Update nightly-readme-build.yml (#31)

Only commit `.md` file

.github/workflows/nightly-readme-build.yml

@@ -34,5 +34,5 @@ jobs:
           Rscript -e 'rmarkdown::render("README.Rmd")'
           git config --local user.name "$GITHUB_ACTOR"
           git config --local user.email "[email protected]"
-          git commit README.* -m 'Re-build Rmarkdown files' || echo "No changes to commit"
+          git commit README.md -m 'Re-build Rmarkdown files' || echo "No changes to commit"
           git push origin || echo "No changes to commit"