Update collaborators section

[embs]

The current link in our collaborators section is pointing to the GitHub network page:

The network page is fun to look at but we have a much better page for collaborators here.

So...

Basic idea

Change the link in the collaborators section from

https://github.com/frevo-on-rails/frevo-on-rails.github.com/network

to

https://github.com/frevo-on-rails/frevo-on-rails.github.com/graphs/contributors

Further improvements

We may also want to make our collaborators section look as nice as the pact-rb one, for instance:

Pretty cool, huh?

While looking for how to add a contributors section to the README, I've also stumbled upon this allcontributors.org/ stuff but I don't really know how easy/viable that would be for us.

Notice

Please don't let the ideas for further improvements block you. It's completely fine if you just update the link and then we think about those ideas later.