Add UPGRADE file for 2.0

Author: alcaeus

UPGRADE-2.0.md

@@ -0,0 +1,11 @@
+# UPGRADE FROM 1.x to 2.0
+
+## Backward compatibility breaks
+
+### Return types
+
+All methods and interfaces have now properly defined return types. These
+correspond to the types previously present in the `@return` attribute of each
+method. A notable exception to this are `MongoDB\Model/CodecCursor::getId` and
+`MongoDB\ChangeStream::getCursorId`, which now return a `MongoDB\BSON\Int64`
+instance.