Prepare for new reactive docs

- Skeleton for migration guide.
- Relates to #742

docs/src/reference/asciidoc/appendix-reactormigration.adoc

@@ -0,0 +1,9 @@
+[appendix]
+[[appendix-reactormigrationguide]]
+== Reactor Migration Guide
+
+Main task for a work for `3.x` has been to both internally and externally to move and change
+as much as we can from imperative code into a reactive world. This means that some
+of a main interfaces has added a new reative methods and most of a internal execution locig
+has been moved over to handled by a reactor. Essentially what this means is that thread handling
+is considerably different compared to `2.x`. Following chapters go throught all these changes.

docs/src/reference/asciidoc/appendix.adoc

@@ -8,3 +8,5 @@ include::appendix-support.adoc[]
 include::appendix-concepts.adoc[]
 
 include::appendix-distributedpaper.adoc[]
+
+include::appendix-reactormigration.adoc[]

docs/src/reference/asciidoc/whatsnew.adoc

@@ -64,4 +64,17 @@ Spring Statemachine 2.0.0 includes the following:
 
 == In 3.0
 
-Spring Statemachine 3.0 focuses on a Reactive support.
+Spring Statemachine 3.0.0 focuses on adding a Reactive support. Moving from `2.x` to `3.x` is
+introducing some breaking changes which are detailed in <<appendix-reactormigrationguide>>.
+
+With `3.0.x` we have deprecated all blocking methods which will get removed at some point
+in a future releases.
+
+[IMPORTANT]
+====
+Please read an appendix <<appendix-reactormigrationguide>> carefully as it will steer you
+through a process of migrating into `3.x` for cases we're not handling internallyl.
+====
+
+At this point most of a documentation has been changed to showcase reactive interfaces
+while we still keep some notes around to users still using old blocking methods.