Add a README

27. Run use_readme_rmd() to:

• Create the source file for package home page on Github.
• Add some lines to .Rbuildignore
• Create a Git pre-commit hook to help you keep README.Rmd and README.md in sync.

use_readme_rmd()
# ✔ Writing 'README.Rmd'
# ✔ Adding '^README\\.Rmd$' to '.Rbuildignore'
# • Modify 'README.Rmd'
# ✔ Writing '.git/hooks/pre-commit'