Add links to blog posts explaining erc721

[come-maiz]

Here are two comprehensive posts with nice explanations about erc721:

https://medium.com/blockchannel/walking-through-the-erc721-full-implementation-72ad72735f3c
https://maksimivanov.com/posts/gradient-coin-tutorial/

I think it would be great to include them in this site.

[tomhschmidt]

Yeah, I'm a big fan of this idea. I'll merge the PR if someone wants to make it.

[fulldecent]

@tomhschmidt #31 is such a PR. It adds two entry level articles. And it leaves room to add more articles.

[abbaskiko]

working