Adding a lazy transformer example.

pkg/barback/example/lazy_transformer/README.md

Index: pkg/barback/example/lazy_transformer/README.md
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+This example shows how to write a lazy transformer.
+
+This lazy tranformer implements a ROT13 converter. ROT13 (ROTate 13)
+is a classic substitution cipher which replaces each letter in the source
+file with the corresponding letter 13 places later in the alphabet.
+The source file should have a ".txt" extension and the converted file
+is created with a ".shhhh" extension.
+
+Generally, only transformers that take a long time to run should be made lazy.
+This transformer is not particularly slow, but imagine that it might be used
+to convert the entire library of congress–laziness would then be a virtue.
+
+For more information, see Writing a Lazy Transformer at:
+https://www.dartlang.org/tools/pub/transformers/lazy-transformer.html