add style guide

[simonDos]

The section on style guides is quite short at the moment.
A more extensive description with examples would be better.

This can be highly valuable for newcomers and for the community as a whole!

There are some good style guides already out there that could be incorporated like this one:
https://github.com/0xcert/solidity-style-guide

[maurelian]

We actually have no content about style guides at all right now AFAICT. This could be valuable, perhaps under documentation and procedures?

I'm especially interested in style guides that have linter plugins.

[samsaif87]

Good