Add #51: Read content from markdown file

[giatuongtran9]

_Can only ready from README.md
_I suggest to move those method to Main.js because we only need to show content of the file whenever the users click to that subject. Here, we show all content of all subjects after the page is loaded. This might caught problem later if the amount of .md file get bigger.
_My suggest is to use ComponentWillReceiveProps to load the only subject that users click. For exmaple: when the user click a specific subject, fetch function will get that subject name and attempt to load that only subject

[10-seconds]

We have improved the website. So can you re raised the PR. Sorry for incovience.