Add a changelog to barback.

pkg/barback/CHANGELOG.md

+## 0.12.0
+
+* Add a `Transform.logger.fine` function that doesn't print its messages by
+  default. When using Barback with pub in verbose mode, these messages will be
+  printed.
+
+* `Transformer.allowedExtensions` now supports extensions containing multiple
+  periods, such as `.dart.js`.
+
+* Transforms now pass their primary inputs through to the next phase by default.

[Bob Nystrom, 2014/03/14 00:01:55]

"Transforms" -> "Transformers"

[nweiz, 2014/03/14 00:06:52]

I thought we used "transformer" to refer to the class that knows how to
transform something and "transforms" to refer to the application of a
transformer to a specific primary input.

[Bob Nystrom, 2014/03/14 00:24:17]

SGTM.

+  A transformer may still overwrite its primary input without causing a
+  collision. If a transformer doesn't overwrite its primary input, it may cause
+  it not to be passed through by calling `Transform.consumePrimary`. The primary
+  input will be consumed by default if a transformer throws an error.
+
+* If an input requested with `Transform.getInput`, `Transform.readInput`, or
+  `Transform.readInputAsString` cannot be found, an `AssetNotFoundException`
+  will be thrown. This was always what the documentation said, but previously a
+  `MissingInputException` was thrown instead.
+
+* If a transformer calls `Transform.logger.error`, the transformer will now be
+  considered to have failed after it finishes running `apply()`. This means that
+  its outputs will not be consumed by future transformers and its primary input
+  will not be passed through to the next phase.