Added Changelog.

CHANGELOG-2.x.md

+CHANGELOG for 2.x
+=================
+* Removed `Rdata\FormattableInterface`, all aligned formatting is handled by `AlignedBuilder`.
+* Removed `ZoneInterface`, `ResourceRecordInterface`, and `ZoneBuilderInterface`.
+* `AlignedBuilder::build()` and `ZoneBuilder::build()` are static methods, no need to instantiate the builder.
+* All methods now type-hint the return value.
+* `Zone` implements `IteratorAggregate` and `Countable` so you can loop over `ResourceRecords` using
+`foreach ($zone as $rr){}`.
+* `Zone::expand()` method has been moved to `ZoneBuilder` and renamed as `ZoneBuilder::fillOutZone()`.
+* Methods in `Validator` have been renamed to better reflect what they are validating.