Add release candidate warning to 0.15 to 0.16 migration guide (#2078)

Author: TrialDragon

content/learn/migration-guides/0.15-to-0.16.md

@@ -6,6 +6,10 @@ weight = 11
 long_title = "Migration Guide: 0.15 to 0.16"
 +++
 
+{% callout(type="warning") %}
+This guide is currently for a __Release Candidate__ and is subject to change as we work on and improve it. If you notice any errors or issues within this guide, then please submit an issue at [the Bevy website repository issue tracker](https://github.com/bevyengine/bevy-website/issues).
+{% end %}
+
 The most important changes to be aware of this release are:
 
 - Bevy has reworked its error handling to make it easier to handle `Result`s everywhere. As a result, `Query::single` and friends now return results, rather than panicking.