【User-Defined Components Must Be Capitalized】
When an element type starts with a lowercase letter, it refers to a built-in component like<div> or <span> and results in a string 'div' or 'span' passed to React.createElement. Types that start with a capital letter like <Foo /> compile to React.createElement(Foo) and correspond to a component defined or imported in your JavaScript file.
We recommend naming components with a capital letter. If you do have a component that starts with a lowercase letter, assign it to a capitalized variable before using it in JSX.
For example, this code will not run as expected:
To fix this, we will rename hello to Hello and use <Hello /> when referring to it:
参考：https://facebook.github.io/react/docs/jsx-in-depth.html#user-defined-components-must-be-capitalized
