Added "keep_md" YAML to template

Adding the YAML with "keep_md: true" will prevent RStudio from erasing the *.md file when clicking on the "Knit HTML" button. Some students forget to run knit2html on their latest version or overlook creating an *.md file altogether. This will save students a little time and frustration.
Update README.md

Minor grammar fixes.
Merge pull request #25 from Ripley6811/Assignment-updates

Assignment updates