update changelog

CHANGELOG.md

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 # Changelog
 
+## Unreleased
+
+* Preliminary tree shaking support ([#50](https://github.com/evanw/esbuild/issues/50))
+
+    Bundling now performs tree shaking, which is also known as dead code elimination. Every top-level statement is considered to be a separate part of the file, and unused parts without any side effects are not included in the bundle. This only really affects code using ES6 modules, so make sure you use ES6 modules to take advantage of tree shaking.
+
+    This is the initial release of tree shaking which lands the fundamental mechanism behind it. This release does not include the [various annotations used by the community](https://webpack.js.org/guides/tree-shaking/) to indicate side-effect free code (e.g. `"sideEffects": false` and `/*#__PURE__*/`), so esbuild will likely generate somewhat bigger bundles than other bundlers. Support for these annotations will come in future releases.
+
+* Add the `esm` output format ([#48](https://github.com/evanw/esbuild/issues/48))
+
+    It is now possible to generate a bundle in ES6 module format using `--format=esm`. The generated code uses ES6 import and export statements. This is useful for bundling code to be used as a library, for using in a `<script type="module>` tag in the browser, or for using with node's `--experimental-modules` flag. Note that CommonJS entry points bundled with this format will become a single default export, which is the same way node works.
+
+* Warnings about future syntax are now errors
+
+    This happens when an input file contains newer JavaScript syntax and `--target` is set to an earlier version of JavaScript than the syntax can be transformed to. These most of transforms will be implemented eventually, but for now some are still unimplemented. This was changed from a warning to an error because ignoring these warnings could result in broken code in older browsers, so these messages are more serious than warnings.
+
+* Using bundle-related flags without `--bundle` is now an error
+
+    This leaves the possibility open of using these flags for non-bundle mode in the future. For example, in the future `--format` may also work when not bundling.
+
 ## 0.3.9
 
 * Add the `dataurl` loader ([#107](https://github.com/evanw/esbuild/pull/107))