Update README for v5

Release v4.1.0

v4.0.0: Add null-safety

v3.0.0: Update loose URLs, parse www. in non-loose mode, parse \r

v2.1.0: Add loose URLs, remove www and exclude last period

v2.0.3: Remove print

v2.0.2: Fix minor lint issues

v2.0.1: export defaultLinkifiers

v2.0.0: enable custom linkifiers

v1.0.1, republish to fix maintenance score