changelog for prevent_initial_call

Author: alexcjohnson

CHANGELOG.md

@@ -4,6 +4,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [UNRELEASED]
 ### Added
+- [#1228](https://github.com/plotly/dash/pull/1228) Adds control over firing callbacks on page (or layout chunk) load. Individual callbacks can have their initial calls disabled in their definition `@app.callback(..., prevent_initial_call=True)` and similar for `app.clientside_callback`. The app-wide default can also be changed with `app=Dash(prevent_initial_callbacks=True)`, then individual callbacks may disable this behavior.
 - [#1201](https://github.com/plotly/dash/pull/1201) New attribute `app.validation_layout` allows you to create a multi-page app without `suppress_callback_exceptions=True` or layout function tricks. Set this to a component layout containing the superset of all IDs on all pages in your app.
 - [#1078](https://github.com/plotly/dash/pull/1078) Permit usage of arbitrary file extensions for assets within component libraries