Add zap4echo to README

[karagenc]

I wrote a middleware package to log requests and to recover from panics. It uses zap, my favorite logging package.

[lammel]

Nice, but how does your package differ from the existing github.com/brpaz/echozap just the line below yours?

[karagenc]

echo4zap is highly configurable whereas echozap is not. In addition to that echo4zap has a logger for panic recovery, which is also configurable.

[lammel]

For adding zap4echo I think having both for now seems ok, as zap4echo is more configurable.

@aldas I'd prefer to move the whole third party middlewares seciotn from the README to the echo-contrib project and just add a link in the main repo README to the echo-contrib project. What do you think about that?

[karagenc]

There was a merge conflict. I solved it