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 require each new account to be manually approved. Please register an account and then write an email to accountsupport@archlinux.org to get it approved. 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.