Openzeppelin-contracts: create tutorials for documentation

[maraoz]

[frangio]

A quick update. A couple tutorials have been written by @gustavoguimaraes:

• Setting Up a Bug Bounty Smart Contract with OpenZeppelin
• How To Create Token and Initial Coin Offering Contracts Using Truffle + OpenZeppelin

[elopio]

We have the guides tag in the blog: https://blog.zeppelin.solutions/guides/home
I'm leaving this open because we are planning some improvements on the docs site, so this will be a reminder that it should include a tutorials section or at least a link to these guides.

But I think that it would be better to file individual issues for each tutorial that we want to add to the docs. I'll be back later to close this :)

[nventuro]

We have some guides in the docsite, and the undergoing revamp will make it easier for everyone to contribute to these so-called 'mini-readme' guides.