add(version.txt): Add initial version metadata file.

This file will be referenced from other repos to determine the current
CoreOS version. This replaces the version shell script in coreos-overlay
as the authoritative source so that the manifest repo is the only place
to update this sort of data.

Change-Id: I5607beb5647810a60007ba552cb81d6dd5cdffc7
Merge pull request #2 from marineam/version

add(version.txt): Add initial version metadata file.