Contribution guidelines draft

The first draft of the contribution guidelines, #1.
I think it is more accurate to say, you should not use a pointer JUST to prevent copying a byte or five. My read of the [Go FAQ about pointer vs value](https://golang.org/doc/faq#methods_on_values_or_pointers) is that it is ambivalent or even leans towards "use a pointer" unless you are passing small, simple values.