doc: added io.js v1.0.0 ChangeLog as a summary

Collaboratively edited by many people in
https://code.stypi.com/domenic/iojs-1.0-changelog.md
and includes a summary of changes in each major section of the
project from a user-facing perspective from Node.js v0.10.34
to io.js v1.0.0.

Renamed file to CHANGELOG.md from ChangeLog.md

Fixes: https://github.com/iojs/io.js/issues/339


Reviewed-By: Fedor Indutny <fedor@indutny.com>