Fix compilation of README and ExternalProject examples

[dajiaohuang]

Summary

Fix three compilation errors in the documented consumer examples:

• Use int main() in the README's cat example (GCC rejects void main()).
• Pass the element type before the output container in numbers<int, Ints>(1, 30), matching the public numbers template signature. numbers<Ints> treats the list type as the numeric element type and does not compile.
• Add /include to the ExternalProject example's include directory, so <fplus/fplus.hpp> can be found in the downloaded repository.

No library behavior or compiler requirements change.

Validation

Tested with GCC 13.3.0, -std=c++14, and CMake 3.28.3 on Ubuntu 24.04:

• Reproduced both README compilation failures and the ExternalProject missing-header failure before these corrections.
• Compiled the corrected README snippets; the cat and Collatz outputs exactly match the documented outputs. For Collatz, supplied the declared std::list<int> collatz_seq(int) implementation used in test/readme_examples_test.cpp.
• Configured, built and ran the documented ExternalProject consumer including <fplus/fplus.hpp>.
• Configured/built the library, installed to a local prefix, and built/ran a find_package(FunctionalPlus) consumer.
• Built/ran the documented FetchContent consumer, reusing the local source checkout through CMake's source override.
• Ran a small independent consumer against both modular and all-in-one headers: vector transform/filter/fold, strings, map creation, grouping, maybe/result propagation, forward pipelines, and the three Gemstones composition styles.
• git diff --check passes.

The full upstream test suite and other compiler/platform/package-manager matrices were not run; this change only corrects documentation examples.

[Copilot]

🟢 Approval recommended

The three focused documentation fixes match the library API and header layout, with no unresolved issues.

0 open findings

What changed in this PR

Corrects three compilation errors in documented consumer examples without changing library behavior or compiler requirements.

Changes:

• Fixes the README’s main return type and numbers template arguments.
• Points the ExternalProject consumer at the repository’s include directory.

File	Description
README.md	Corrects the cat and Collatz examples.
INSTALL.md	Fixes the ExternalProject include path.

🧠 Review effort: Balanced

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