Add files using use_r and use_test

[SaranjeetKaur]

• Added a new add function to perform addition of two numbers in the R/add.R file.
• Updated the DESCRIPTION file to include testthat and roxygen2 configurations.
• Updated the NAMESPACE file to export the new add function.
• Added documentation for the add function in the man/add.Rd file.)
• Removed the hello function and its documentation.
• Added unit tests for the add function in tests/testthat/test-add.R.
• Created a setup file for testthat in tests/testthat.R.
• Added instructions for adding R files, setting up unit tests, and documentation in the README.Rmd and README.md files.