Rename README.Rmd to README.md

Not sure if it was intentional, but github isn't rendering the README.Rmd file as an .md file.

Please ensure the following before submitting a PR:

[ ] if suggesting code changes or improvements, open an issue first
[ ] for all but trivial changes (e.g., typo fixes), add your name to DESCRIPTION
[ ] for all but trivial changes (e.g., typo fixes), document your change in NEWS.md with a parenthetical reference to the issue number being addressed
[ ] if changing documentation, edit files in /R not /man and run devtools::document() to update documentation
[ ] add code or new test files to /tests for any new functionality or bug fix
[ ] make sure R CMD check runs without error before submitting the PR

cloudyr / aws.athena