Add a simple documented example

[psiphi75]

Added a simple documented example on how to use real-world data and how to extract the results. Only one file was created, this is examples/simple.rs. No other changes were made.

I needed to get my head around how to do this, so I did, and I thought I would provide it.

[Norlock]

Thanks I was already looking for something like this! Since I didn't understand how to use this lib, this is making it really easy.

[WalterSmuts]

If you're trying to provide an example of how to work with real-valued signals I'm not sure it's appropriate here. realfft is a nice wrapper if you know you're only working with real-valued signals.