Update README

[ghost]

I noticed a few typos in the README, so went through and updated it a bit (fixed typos, re-worded some bits, etc.). I also removed the last bit about the Backdrop server and the master branch since this isn't currently the case (there's no automatic pulling from the master branch), and based on Jen's comments in another issue I don't think it's likely to happen (probably we'll just manually update the Backdrop server each release of this project).