Update README.md
Small updates for README.md: update info about mirror.pkgbuild.com as pointed out by @klausenbusk and fix some obvious typos and Markdown formatting errors.
Due to an influx of spam, we have had to temporarily disable account registrations. Please write an email to accountsupport@archlinux.org, with your desired username, if you want to get access. Sorry for the inconvenience.
Small updates for README.md: update info about mirror.pkgbuild.com as pointed out by @klausenbusk and fix some obvious typos and Markdown formatting errors.