Revert "remove serialization. it's moved to github"

pkg/serialization/README.md

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+A general-purpose serialization facility for Dart Objects.
+
+This provides the ability to save and restore objects to pluggable
+Formats using pluggable Rules.
+These rules can use mirrors or be hard-coded. The main principle
+is using only public APIs on the serialized objects, so changes to the
+internal representation do not break previous serializations. It also handles
+cycles, different representations, filling in known objects on the 
+receiving side, and other issues. It is not as much intended for
+APIs using JSON to pass acyclic structures without class information,
+and is fairly heavweight and expensive for doing that compared to simpler
+approaches.