Added a changelog

CHANGELOG.md

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+# Change Log
+All notable changes will be documented in this file.
+Swagger Parser adheres to [Semantic Versioning](http://semver.org/).
+
+
+## [v3.3.0](https://github.com/BigstickCarpet/swagger-parser/tree/v3.3.0) (2015-10-02)
+
+Updated to the latest version of [the Official Swagger 2.0 Schema](https://www.npmjs.com/package/swagger-schema-official).  The schema [hadn't been updated for six months](https://github.com/swagger-api/swagger-spec/issues/335), so Swagger Parser was missing [several recent changes](https://github.com/swagger-api/swagger-spec/commits/master/schemas/v2.0/schema.json).
+
+[Full Changelog](https://github.com/BigstickCarpet/swagger-parser/compare/v3.2.0...v3.3.0)
+
+
+## [v3.2.0](https://github.com/BigstickCarpet/swagger-parser/tree/v3.2.0) (2015-10-01)
+
+Swagger Parser now uses [call-me-maybe](https://www.npmjs.com/package/call-me-maybe) to support [promises or callbacks](https://github.com/BigstickCarpet/swagger-parser/tree/master/docs#callbacks-vs-promises).
+
+[Full Changelog](https://github.com/BigstickCarpet/swagger-parser/compare/v3.1.0...v3.2.0)
+
+
+## [v3.1.0](https://github.com/BigstickCarpet/swagger-parser/tree/v3.1.0) (2015-09-28)
+
+Fixed several bugs with circular references, particularly with the [`validate()`](https://github.com/BigstickCarpet/swagger-parser/blob/master/docs/swagger-parser.md#validateapi-options-callback) method.
+
+Added a new [`$refs.circular` option](https://github.com/BigstickCarpet/swagger-parser/blob/master/docs/options.md) to determine how circular references are handled.  Options are fully-dereferencing them (default), throwing an error, or ignoring them.
+
+[Full Changelog](https://github.com/BigstickCarpet/swagger-parser/compare/v3.0.0...v3.1.0)
+
+
+## [v3.0.0](https://github.com/BigstickCarpet/swagger-parser/tree/v3.0.0) (2015-09-25)
+
+This is a **complete rewrite** of Swagger Parser.  Major changes include:
+
+**Swagger 2.0 Compliant**<br>
+Previous versions of Swagger Parser were based on early drafts of Swagger 2.0, and were not compliant with [the final version of the spec](https://github.com/swagger-api/swagger-spec/blob/master/versions/2.0.md).  Swagger Parser v3.0 is now compliant with the final spec as well as related specs, such as [JSON Reference](https://tools.ietf.org/html/draft-pbryan-zyp-json-ref-03) and [JSON Pointer](https://tools.ietf.org/html/rfc6901)
+
+**All-New API**<br>
+The old API only had a single method: `parse()`.  But depending on which options you passed it, the method did _much_ more than its name implied.  The new API has separate methods to make things a bit more intuitive.  The most commonly used will be [`validate()`](https://github.com/BigstickCarpet/swagger-parser/blob/master/docs/swagger-parser.md#validateapi-options-callback), [`bundle()`](https://github.com/BigstickCarpet/swagger-parser/blob/master/docs/swagger-parser.md#bundleapi-options-callback), and [`dereference()`](https://github.com/BigstickCarpet/swagger-parser/blob/master/docs/swagger-parser.md#dereferenceapi-options-callback).  The [`parse()`](https://github.com/BigstickCarpet/swagger-parser/blob/master/docs/swagger-parser.md#parseapi-options-callback) and [`resolve()`](https://github.com/BigstickCarpet/swagger-parser/blob/master/docs/swagger-parser.md#resolveapi-options-callback) methods are also available, but these are mostly just for internal use by the other methods.
+
+**Asynchronous API**<br>
+The old API was "asynchronous", but it relied on global state, so it did not support multiple simultaneous operations.  The new API is truly asynchronous and supports both [ES6 Promises](http://javascriptplayground.com/blog/2015/02/promises/) and Node-style callbacks.
+
+**New JSON Schema Validator**<br>
+Swagger Parser has switched from [tv4](https://github.com/geraintluff/tv4) to [Z-Schema](https://github.com/zaggino/z-schema), which is [faster](https://rawgit.com/zaggino/z-schema/master/benchmark/results.html) and performs [more accurate validation](https://github.com/ebdrup/json-schema-benchmark#test-failure-summary).  This means that some APIs that previously passed validation will now fail.  But that's a _good_ thing!
+
+[Full Changelog](https://github.com/BigstickCarpet/swagger-parser/compare/v2.5.0...v3.0.0)