Simplified author list in README.md

Instead of filtering the complete git log, `git shortlog -s` (‘s’ for “summary”) can be used to view a list of authors with the number of their respective commits. `git shortlog` also has the benefit of respecting a .mailmap file if it exists, which allows to fix incorrect e-mails or names.
 In addition, Github also offers a list of contributors.

(I hope the Unicode characters in the message are okay?)