Add a "Hello, world!" example.

doc/examples/hello/.gitignore

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+# CMake artifacts
+build/
+CMakeUserPresets.json

doc/examples/hello/CMakeLists.txt

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+cmake_minimum_required(VERSION 3.15)
+project(hello CXX)
+find_package(know.cpp REQUIRED)
+add_executable(${PROJECT_NAME} src/main.cpp)
+target_link_libraries(${PROJECT_NAME} know.cpp::know.cpp)

doc/examples/hello/README.md

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+# Hello, World! for Know.cpp
+
+```console
+$ conan install . --output-folder=build --build=missing
+
+$ cd build
+$ cmake .. -DCMAKE_TOOLCHAIN_FILE=conan_toolchain.cmake -DCMAKE_BUILD_TYPE=Release
+$ cmake --build .
+
+$ ./hello
+```

doc/examples/hello/conanfile.txt

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+[requires]
+know.cpp/0.0.0
+
+[generators]
+CMakeDeps
+CMakeToolchain

doc/examples/hello/src/main.cpp

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+/* This is free and unencumbered software released into the public domain. */
+
+#include <know.hpp> // for know::*
+
+#include <cstdlib>  // for EXIT_SUCCESS
+#include <iostream> // for std::cout
+
+int main(void) {
+  know::thing you;
+  you.names = {"J. Random Hacker"};
+
+  std::cout << "Hello, " << you.names[0] << "!\n";
+
+  return EXIT_SUCCESS;
+}