The idea is that we add files to the api/ directory which
are sets of promises for the future.  Each line in a file
is a stand-alone feature description.

When we do a release, we make sure we haven't broken or changed
any lines from the past (only added them).

We never change old files, only adding new ones. (go-1.1.txt,
etc)

R=dsymonds, adg, r, remyoudompheng, rsc
CC=golang-dev
https://golang.org/cl/5570051