add principal changes to readme

Author: axtimhaus

README.md

@@ -15,6 +15,21 @@ Version 1.0 will be out of support, as version 2.0 is released.
 Consider to start development targeting version 2.0 as soon as possible.
 The API of version 2.0 is mostly fixed, but there may be minor but breaking changes until release.
 
+### Principal Changes of Version 2.0 Respective to 1.0
+
+- Split of old `pyroll` package into `pyroll-core`, `pyroll-report`, `pyroll-export` and `pyroll-cli`.
+- Complete reworking of the hook system.
+  - Implementation of own hooking framework and removal of pluggy.
+  - Simpler user interface for hook definition and implementation.
+  - More orthodox default hook implementations, removal of "basic models".
+- Rework of the object model.
+  - Stricter hierarchy of unit classes.
+  - Extended possibilities of groove definition.
+  - Roll passes with three working rolls.
+  - New unit classes: `Rotator`, `PassSequence`, `ThreeRollPass`.
+  - Introduction of disk elements for incremental modelling in rolling direction.
+  - Nestable units (esp. pass sequences and disk elements).
+
 ## Documentation
 
 See the [documentation](https://pyroll.readthedocs.io/en/latest) to learn about basic concepts and