Update README.md

Support multi-character custom delimiters (e.g. //[***]\n)

Add test for multiple negative numbers in error message

Raise error when negative numbers are provided

Support custom single-character delimiters using //delimiter syntax

Added to support for newlines as delimiters

Add test for handling multiple numbers (no code change needed here)

Support addition of two comma-separated numbers

Handle single number input

Add test and implementation for empty string returning 0

Initialize Ruby project,RSpec setup,lib directory