doc/asm.html: new document, a brief guide to the assembler

Fixes issue 6060

R=golang-dev, iant, bradfitz, josharian, minux.ma, aram, rsc
CC=golang-dev
https://codereview.appspot.com/20930043
src/cmd/?a: link to new assembler document
Blocked on 20930043, the CL the new text references.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/18430044