docs: add CMake build support

[nouscow]

Summary

• Add CMakeLists.txt for cross-platform CMake-based build
• Add "Build with CMake" section to README
• Documents direct g++ compilation as alternative

Motivation

Users (especially beginners) often struggle with manually linking pthread.
A standard CMakeLists.txt with find_package(Threads) makes integration
much easier across Linux, macOS, and Windows.

Files changed

File	Change
CMakeLists.txt	New — builds example.cpp with C++11 + Threads
README.md	Added build instructions
.gitignore	Ignore build artifacts and IDE configs