Update CHANGELOG

Author: duckontheweb

CHANGELOG.md

@@ -82,7 +82,7 @@
 ### Changed
 
 - API change: The extension API changed significantly. See ([#309](https://github.com/stac-utils/pystac/pull/309)) for more details.
-- API change: Refactored the global STAC_IO object to an instance-specific `StacIO` implementation. STAC_IO is deprecated and will be removed next release. ([#309](https://github.com/stac-utils/pystac/pull/309))
+- API change: Refactored the global STAC_IO object to an instance-specific `StacIO` implementation. ([#309](https://github.com/stac-utils/pystac/pull/309))
 - Asset.get_absolute_href returns None if no absolute href can be inferred (previously the relative href that was passed in was returned) ([#309](https://github.com/stac-utils/pystac/pull/309))
 
 ### Removed
@@ -91,6 +91,11 @@
 - Removed `LinkMixin`, and implemented those methods on `STACObject` directly. STACObject was the only class using LinkMixin and this should not effect users ([#309](https://github.com/stac-utils/pystac/pull/309)
 - Removed `single-file-stac` extension; this extension is being removed in favor of ItemCollection usage ([#309](https://github.com/stac-utils/pystac/pull/309)
 
+# Deprecated
+
+- Deprecated `STAC_IO` in favor of new `StacIO` class. `STAC_IO` will be removed in
+  v1.0.0. ([#309](https://github.com/stac-utils/pystac/pull/309))
+
 ## [v0.5.6]
 
 ### Added