Fix the README

[Joonho-Hwang]

In the Serialize custom type example, write and read function should be a template specialization.
So,

// Let's have a 3D vector struct:
struct vec3 { float x, y, z; };

// Let's have a triangle struct with 'vec3' members
struct Triangle
{
	vec3 a, b, c;
};

JSON5_CLASS(Triangle, a, b, c)

namespace json5::detail {

// Write vec3 as JSON array of 3 numbers
template<>
inline json5::value write( writer &w, const vec3 &in )
{
	w.push_array();
	w += write( w, in.x );
	w += write( w, in.y );
	w += write( w, in.z );
	return w.pop();
}

// Read vec3 from JSON array
template<>
inline error read( const json5::value &in, vec3 &out )
{
	return read( json5::array_view( in ), out.x, out.y, out.z );
}

} // namespace json5::detail

This is the right way to do this.

Also, please make a note about the #include order. It seems like json5_input.hpp should be included in prior to json5_reflect.hpp.