Add explanation to secp256k1 field.go

I was reading through the code, on how the keys are generated and came to with [this](https://github.com/skycoin/skycoin/blob/develop/src/cipher/secp256k1-go/secp256k1-go2/field.go#L62), the function (or the entire field.go) could really use an explanation on what's beeing done here, or at least a reference to a document.