doc/go1.3.txt: add file to track significant CLs for 1.3 Nothing significant so far (right?), but let's lay the keystone.
doc/go1.3.txt: add file to track significant CLs for 1.3
Nothing significant so far (right?), but let's lay the keystone.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/36830043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang