Add a guide to build an upgradeable ERC20

[facuspagnuolo]

One of the basic examples we can add to the documentation site is one to show how to build an upgradeable ERC20 token. It may be interesting to add a section of how you can do it using openzeppelin-eth to avoid writing it from scratch.

We can show further examples of minting some tokens on the initialization, upgrading it to anothe version, transferring tokens, transferring control of it, etc

[eternauta1337]

Isn't this solved already? See: https://docs.zeppelinos.org/docs/erc20_onboarding.html

[spalladino]

@ajsantander no, that guide is on migrating an existing ERC20 token into an upgradeable one. This issue is about creating a new one from scratch, so should be much more simple. Still, we can defer it for later, no need to tackle this issue now.

[crazyrabbitLTC]

Is this still something we want to do?

[spalladino]

Certainly! Not high prio, but definitely a nice addition to our guides. We could also replace the linking EVM packages guide by this one, to better promote the link between OZ and zOS. Anyway, just a blogpost or forum article is also fine.