Bylaw Errata explanation and PSR-2 Errata

After the [Errata Vote](https://groups.google.com/forum/?fromgroups=#!topic/php-fig/qTROKw07848) passes we will be able to add Errata to PSRs. This PR adds the wording to the Workflow Bylaw, and adds a PSR-2 Errata document.

I'm rolling these two things together as the intention of the Poll and the Vote for Errata has always made it extremely clear that PSR-2 Errata was the intention, so it should come as no surprise that these are both happening. I don't want to wait two weeks to get the Errata bylaw change voted in THEN another two weeks to get this specific piece of errata in.

![image](https://f.cloud.github.com/assets/67381/1103967/9aa65408-18d3-11e3-8928-4d6fdf6c0b78.png)