Moving up to a major version (1.0) implies there could have been breaking changes if you follow semver. And including something like `unnest_legacy()` is helpfu

Moving up to a major version (1.0) implies there could have been breaking changes if you follow semver. And including something like `unnest_legacy()` is helpful for people making the transition. Just like `stringsAsFactors=FALSE` happened on a transition from 3.x to 4.x, because it is breaking, dplyr 1.0 had breaking changes.