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

tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang