Move httplibConf.cmake.in & install readme/license (yhirose#1826)

* Move httplibConfig.cmake.in to cmake dir

Just makes more sense to put it there I suppose.

* Cmake install README & License

Seems to make sense since you might already do this as a package
manager, or an end user might want them anyways.

The locations are just based on standard Linux locations using
GNUInstallDirs, so it should be sane on other machines too.

CMakeLists.txt

@@ -245,7 +245,7 @@ set(_TARGET_INSTALL_CMAKEDIR "${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}")
 include(CMakePackageConfigHelpers)
 
 # Configures the meta-file httplibConfig.cmake.in to replace variables with paths/values/etc.
-configure_package_config_file("${PROJECT_NAME}Config.cmake.in"
+configure_package_config_file("cmake/${PROJECT_NAME}Config.cmake.in"
 	"${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}Config.cmake"
 	INSTALL_DESTINATION "${_TARGET_INSTALL_CMAKEDIR}"
 	# Passes the includedir install path
@@ -293,6 +293,11 @@ if(HTTPLIB_INSTALL)
 		NAMESPACE ${PROJECT_NAME}::
 		DESTINATION ${_TARGET_INSTALL_CMAKEDIR}
 	)
+
+	# Install documentation & license
+	# ex: /usr/share/doc/httplib/README.md and /usr/share/licenses/httplib/LICENSE
+	install(FILES "README.md" DESTINATION "${CMAKE_INSTALL_DOCDIR}")
+	install(FILES "LICENSE" DESTINATION "${CMAKE_INSTALL_DATADIR}/licenses/${PROJECT_NAME}")
 endif()
 
 if(HTTPLIB_TEST)