Contribution guidelines draft

The first draft of the contribution guidelines, #1.
From this quote, my view was the preference is with using values except with large `struct`s which would be rare and fall under the "have to" :

> For types such as basic types, slices, and small structs, a value receiver is very cheap so unless the semantics of the method requires a pointer, a value receiver is efficient and clear.

I was trying to convey the points made in the link with few words, maybe it can be reworded but I find the quote above is a good rule of thumb, it just needs to be reworded and the link will give extra guidance.

How about this?

> Pass values, unless the value is a large `struct` or the semantics of the method requires a pointer.

Also the link is pointing to the wrong section, it should point here :
https://github.com/golang/go/wiki/CodeReviewComments#receiver-type