add CHANGELOG.md

I thought adding this change log might be helpful to anyone who's in the process of upgrading node-webkit. I recently found myself upgrading from 0.7.2 to 0.9.2, and had to go through the process of looking up each "upgrade" post on the Google Group to get a full picture of what I was getting myself into upgrade-wise.

This is a first draft. I simply have a version number paired with the release date (which I took from each announcement post's timestamp) followed by a bulleted list of changes. For the changes, I just pretty much copy-pasted the content from each version's respective announcement post. I did some light formatting and summarization of larger changes, especially on the major 0.x.0 releases. 

I went only as far back as version 0.6.0, but I could go farther if you want.