docs: reword project messaging

Rewords project messaging in README to make the overall project
messaging a bit clearer. More discussion to be found in iojs/io.js#24.

PR-URL: https://github.com/iojs/io.js/pull/36
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>