json: Add package comment
json: Add package comment

R=golang-dev, r2, r
CC=golang-dev
http://codereview.appspot.com/2149047

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

h8liu/golang,gnanderson/go,tv42/old-go,dsriram/golang,apparentlymart/golang,apparentlymart/golang,dsriram/golang,ssrl/go,ssrl/go,h8liu/golang,dsriram/golang,apparentlymart/golang,ssrl/go,h8liu/golang,gnanderson/go,gnanderson/go,tv42/old-go,tv42/old-go,dsriram/golang,ssrl/go,gnanderson/go,apparentlymart/golang,dsriram/golang,tv42/old-go,ssrl/go,pombredanne/go-deleteme,ssrl/go,dsriram/golang,h8liu/golang,apparentlymart/golang,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,h8liu/golang,gnanderson/go,ssrl/go,pombredanne/go-deleteme,apparentlymart/golang,h8liu/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,gnanderson/go,tv42/old-go,h8liu/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang