Enhance README with usage and library consumption details

[sean-parent]

Issue #6: Updated usage instructions and added a section on consuming libraries with cpp-library.

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Note

Expands README with CPM-based usage and consumption examples and documents target naming/alias behavior.

• README:
  • Usage: Clarifies CPM installation and fetching via CPMAddPackage.
  • Library Consumption: Adds "Using Your Library" section with CPMAddPackage and target_link_libraries example.
  • Target Naming: Documents primary target from project() and alias via NAMESPACE, including namespace-stripping rule for namespace-foo -> namespace::foo.

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