Fix grammar and spelling in readme

Hi @haya14busa, this is a fix for the readme's spelling and grammar (issue #80). I may have gone a little overboard with the edits, but overall I think you will be happy with the changes I made. The biggest thing was the verb tense; it's not actually clear with the original wording that the current version *is* 2.0. Changing to the present and/or present progressive tense is better. Most of the other edits were just to improve the clarity of the writing.