README: document the plain CMake install route

[Bronek]

Gap

README.md ## Using the library → As a dependency opens with

find_package(libfn CONFIG REQUIRED)
target_link_libraries(main PRIVATE libfn::fn)

but the document never states the most direct way to make that line resolvable without a registry: installing the package from a source checkout. The packaging sentence lists conan, vcpkg, Nix, Bazel, and "plain CMake FetchContent or add_subdirectory" — the latter two are the sub-project route, not the install route. The install tree already exports a complete, relocatable config-mode package (libfnConfig.cmake, libfnConfigVersion.cmake, libfnTargets), so this is purely a documentation gap: the capability exists and is exercised by the registries; only the direct consumer is left uninformed.

Proposed wording

Insert in ### As a dependency, immediately after the packaging-routes sentence (currently README.md:175) and before the compile-options paragraph:

The package can also be installed straight from a source checkout or release tarball:

cmake -B .build -DLIBFN_TESTS=OFF
sudo cmake --install .build    # `sudo` required because install prefix is default `/usr/local`
# cmake --install .build --prefix ~/.local # install in user directory

after which find_package(libfn CONFIG REQUIRED) works as above (for a non-default prefix, point CMAKE_PREFIX_PATH at it).

Details the wording deliberately encodes:

• -DLIBFN_TESTS=OFF: tests default to ON even when libfn is the top-level project, so a bare configure fetches Catch2 and a bare install builds the whole suite first. With tests off, a header-only install compiles nothing.
• The config package is generated via configure_package_config_file, so it is relocatable — cmake --install .build --prefix <path> is safe without reconfiguring.
• .build follows the repository's own build-tree convention (.build* is gitignored, as is build/), so the documented commands leave a cloned checkout clean.
• Placement before the compile-options paragraph is deliberate: the installed package exports the same INTERFACE compile options as every other CMake route, so the existing "every packaging route above except Bazel" sentence covers this route as-is once it is listed above it.

Assisted-by: Claude:claude-fable-5