Add a piece of documentation for correlate

[cwayfinder]

Thanks for your great stuff.

Basically, I got the idea behind the correlate module, but it'd be great to have some explanation in the documentation or some short example.

[timdeschryver]

There were docs, but I removed them for now.
This, because the API might change and I want to test it out first.

See https://github.com/timdeschryver/ngrx-etc/blob/fd9305a126a95d9427e297721ae7a842dd738f76/README.md#correlation for the original docs.

Feel free to use correlate if you'd like, I'm happy to hear some feedback about the functionality it provides.