Added R-cheat sheet

[MrTejas]

Contributing

If you want to add a project to this list, please make sure that:

• [ ] You read the contributing guide
• [ ] The repository has hacktoberfest as topic and counts for hacktoberfest event
• [ ] The project can't be just an "add your name" or simply + 1 contribution to hacktoberfest
• [ ] Place them in alphabetical order
• [ ] One repository per PR/contribution - avoid conflicts and simplify review
• [ ] Your pull request is not spammy just adding an empty line, otherwise it will be marked as invalid and not count to hacktoberfest.

If they meet the requirements above, feel free to create a PR. Happy Hacking! :sparkles: