Update README.md

[marcmerlin]

Address #1748

[hzeller]

Looks good, but we also need to update the COPYING file and all license information at the top of every source file.

[marcmerlin]

@hzeller while this is technically right and ideally better if you're wanting to do it, I think this should bring us 95% of the way there (clearly stating intent and that will be good enough for most)
Are you ok if I merge this, which would not prevent a future update to all the files if needed/desired?