README is outdated (describes state of 2015)

[jonassmedegaard]

README.md describes the landscape, but as it looked 5 years ago.
Firefox implemented Unified Plan "as of recently" - in March 2015.
Chromium-based browsers transitioning from Plan B to Unified Plan still "take some time" but referenced bug about implementing Unified Plan is now done (even if not yet formally closed) and it makes sense to instead (or also?) link to https://bugs.chromium.org/p/chromium/issues/detail?id=857004 about making it the default.