Create What's-different-in-3.0.md

Author: akarnokd

docs/What's-different-in-3.0.md

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+Table of contents
+
+# Introduction
+TBD.
+
+### API signature changes
+
+TBD.
+
+- as() merged into to()
+- some operators returning a more appropriate Single or Maybe
+- functional interfaces throws widening to Throwable
+- standard methods removed
+- standard methods signature changes
+
+### Standardized operators
+
+(former experimental and beta operators from 2.x)
+
+TBD.
+
+### Operator behavior changes
+
+TBD.
+
+- connectable sources lifecycle-fixes
+
+
+### Test support changes
+
+TBD.
+
+- methods removed from the test consumers