Add brief summary of memory alignment to README

[enocom]

This is a great project. Thank you for sharing it with the community.

I discovered the project when gometalinter ran against some of my code and found an inefficient alignment in one of my code's structs. But when I looked up the project, I found nothing in the README or the GoDoc that explained why memory alignment is a good thing or how I might achieve it.

I read Padding is hard which helped, but wonder if we might add something in the README or even the source code to help others who might have the same problem.

/cc @markmandel