Add CMake build instructions for macOS, Windows and Linux including
requirements and installation steps.

### Motivation:

Document how to build SwiftLog with CMake for projects that use mixed
build systems and need integration outside of Swift Package Manager.

### Modifications:

- Added a new “Building with CMake” section to `README.md`
- Documented minimum requirements:
  - Swift 6.1+
  - CMake 3.19+
  - Ninja 1.10+
- Added macOS build examples for both Xcode and Ninja generators
- Added Windows build instructions using Ninja
- Added install instructions and notes about shared vs. static builds
- Clarified that tests are not included in the CMake build and are still
managed through Swift Package Manager


### Result:

Users now have platform-specific guidance for building SwiftLog with
CMake on macOS, Windows and Linux.

Fixes #492