Release note review

Review edits for https://github.com/python-pillow/Pillow/pull/1032

 * English changes.
 * Renamed filename to 2.7.0 because we may have a 2.7.1.
 * Similarly, refer to 2.7.0 in the document rather than 2.7, and "Pillow 2.7.0" just so there's no confusion with Python 2.7.x.


Both are fine. The performance measurements were measured against only the previous version, not all previous versions.