Sample code and output for basic operators

I had spent weeks before finally understood how exactly do basic operators work and why do they exist. I created this post for my own blog, but maybe it can be useful for other newcomers here, in documentation. So please feel free to reject this patch. No matter what you decide, any feedback is highly appreciated. Thanks in advance!

[Rendered Markdown](https://github.com/shpakovski/ReactiveCocoa/blob/patch-1/Documentation/BasicOperators.md)